I creep toward the edge for a closer look at the nesting boobies near the cliff's edge. The enormous brown birds have made their home here for centuries: Cayman Brac settlers first created this trail in the 18th century to collect booby eggs. Erin shouts out, "Sister, Stop!" I do, appearing obedient. But in truth, the sensation of falling had surged through me the moment I'd caught sight down the cliff of the waves crashing against its base. I scramble back towards Erin and we continue down the coral trail, imagining a time when this very place was submerged, when it may have resembled the incredible walls we've been diving on all week in the Bloody Bay Marine Park in the Brac's little sister, neighboring Little Cayman.

When I heard the term Sister Islands, used to refer to Little Cayman and Cayman Brac — the tiny nature-rich gems 90 miles east of the diver's mecca of Grand Cayman — I knew I had to take Erin there with me. After all, it was she who got me started diving, and together we share many travel memories: Turning cartwheels down the mysterious underwater Bimini Road, believed by some to be the lost road to Atlantis; me searching for her, last seen in a bikini and sarong in a Portuguese fishing village at 3 a.m.; and most recently, diving with our newly certified mom in Belize. But I fear that's all about to end. In a few months Erin will remarry, resulting in a blended family with three children. Commitments will be reprioritized: Sister (that's me) will fall down the list. 'Tis life, I understand. So while most people celebrate a final hurrah amidst Vegas-like hedonism, Erin and I looked to the Sister Islands — Little Cayman's legendary walls and Cayman Brac's natural beauty — to affirm our own friendship and to help keep us close as life changes.

Diving In For me, the name Mixing Bowl conjures many images (cookie dough, for instance). But in Bloody Bay Marine Park it is the legendary site where swim-through-friendly Jackson Wall with its Swiss-cheese-like substrate meets the skyscraper-sheer Bloody Bay Wall. And if it sounds cool to divers, for some reason it's even more interesting to the marine life that schools at this intersection of topographical contrasts. As it turns out, this and all the rest of my favorite dives are within half a dozen moorings of each other: Marilyn's Cut, Dottie's Delight, Randy's Gazebo, Lili's. The names are to divers what the Social Register is to blue bloods. By day three with Reef Divers I'm certain that this is some of the best diving in the Caribbean. Mixing Bowl is in one of the shallowest sections of Bloody Bay Wall. Erin and I translate that to mean one thing, extended bottom time, and we are keen to explore. This site is a fun house, and the options are endless: Erin waits for me at the end of one sand-chute swim-through, and as we start back through another, we discover the passage filled by an enormous sleeping nurse shark. One moment we're watching a hawksbill turtle perform its graceful dance; the next a reef shark flies towards us from the shallows and then sails overhead, disappearing into the blue. Grouper line up at cleaning stations and a secretary blenny catches my eye — and my extended attention — on a coral head out on the wall. As soon as we emerge from the water, all we can talk about is getting back in. Erin and I agree that we could dive Mixing Bowl every day and never get bored, though frankly it's always been Erin who brings playfulness and joy to each underwater discovery. I could dive anywhere with her and never get bored.

allie oops! When the boat pulls into Little Cayman Beach Resort, Erin and I race up to our room to shower and change in time to watch the parade. Jim, the resort's manager, had made sure we knew about it the night before while he served as MC of karaoke night at the Tipsy Turtle, the hotel's outdoor bar. Today is Mardi Gras on Little Cayman and soon we discover that most of the island's 100-plus residents — iguanas do outnumber people nearly 20 to 1 — are in the parade. This is no show-me-your-?#%!@ kind of parade, but rather one where floats are filled with mermaids and mermen, where the island's four babies are centerpieces of elaborate tableaus created around their prams (Jim's toddler is a diver in the making, complete with plush-toy fish and sharks dangling from strings and slippers fashioned as fins), where a group of women adorned with fairy dust and gossamer butterfly wings appear as though they may take flight. We are witnessing a community engaged in their fantasy of island life. I wonder what it would be like to have made the decisions, to have ridden the current, that led them here. We follow alongside the parade collecting beads until it ends at the big parking lot beside the "airport" and the lovely plantation-house-style cottage that houses the Hungry Iguana, the restaurant/bar that for now is mostly filled with sea breeze. Out back we spot a large swing hanging from a sea-grape tree and can't resist. No sooner have we settled aboard when the seat flips backward, our skulls smacking the hardpan that's hidden beneath a thin layer of sand. Shocked silence, a few tears, then hysterical laughter. The barmaid runs out with bags of ice for our heads, and Erin and I soothe our nerves (and our egos) with a round of Cayman-brewed Stingray beer on the covered porch overlooking the sea. Beside us are the butterfly women, now toting a trophy. A Date With Jerry Head bumps or not, there's no doubt we're diving in the morning when we return to Bloody Bay Wall on the north side of the island. On deck is Marilyn's Cut, swimming distance west of Mixing Bowl. That's where the groupers named Jerry and Mini-Me live, both known for their tameness around divers. The story has it Jerry was once Mini-Me to a larger grouper named Ben, who found his way onto someone's dinner plate. So now it's Jerry, a three-foot Nassau grouper, who has assumed the senior position. It's time to find out for ourselves. We cruise down the wall, which is encrusted with rope, pipe and azure vase sponges, descending to 50 feet where a giant grouper — no doubt Jerry — swims directly to us, often coming within arm's length, allowing itself to be gently touched by outstretched hands. Erin eventually swims off, and she's inspecting something on the wall when Jerry swims up beside her. Diver. Grouper. Both are entranced; Erin knows she is privileged to be here, now, observing whatever is hiding on the wall, while opportunistic Jerry is hoping for an easy meal. It's a long while before he gives up and swims away. Regretfully, we ascend to the shallows.

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There, our divemaster, Dottie, in a psychedelic-colored dive skin, is rubbing Mini-Me's chin for a small group of divers. We back away from the crowd and fin back over the ledge. There's Jerry, luring us back down the wall. The next 10 minutes are magic. A moray eel is hunting in broad daylight, its body a ribbon weaving in and out of shadows; a pair of reef sharks cruise past; a turtle captivates; and a French angel swims right up to my mask, as though checking to see if I'm OK. Erin and I are positively giddy about this unexpected gift — a string of moments, each better than the next. Given our wonky profile, we stick to the sun-dappled shallows the next dive (cleaning stations and macro galore!) and then take the afternoon off to join LCBR manager Jim and his family plus a few other guests on an outing to Sandy Point, at the far east of the island. There we find a white-sand beach flecked with pink, like the confetti cakes of our childhood birthdays, surrounded by some A+ snorkeling inside the reef: We find enormous lobsters, tons of reef fish and healthy coral that's free of sand. A small group of expats are barbecuing beside the single thatched palapa and we join them in the shade for a while. What makes a couple from the Midwest pack it all in and become Caymanians (or, as I like to say, Caymaniacs)? The answer, I soon realize, is exactly what we're experiencing: days spent on and beneath the gin-clear water, luxuriating on this pillow-soft beach that juts into the sea, fresh seafood, days passed without ever wearing shoes. I know we both want to stay longer. We'd make a party right here on the beach, shop for provisions and invite anyone we meet with a good vibe to join us. But we have plane tickets booked and spots reserved on dive boats on Cayman Brac. I watch Erin comb the beach for shells and pick up my pace so I can check out her finds. Little Cayman has already shared her gifts. I wonder what her sister will give us.

A Family Resemblance The flight to Cayman Brac lasts barely eight minutes. Sandy Point — stunning in its turquoise and white perfection — is the last we see of Little Cayman, and for a few moments I question our plan to move on. Arriving after the morning dive boats have departed, I pick up the courtesy phone in the lobby of Divi Tiara Beach Resort and within 10 minutes a rental car is delivered. We plan to keep it for a day, but soon find that even on an island with so little to do, there's always somewhere we want to go. The car stays with us until we leave. So here we are, heading for the hills — or rather the lighthouse at the crest of the bluff at the far east of the island. I take the wheel on the right side of the car (remember, we're in a British territory) and Erin navigates. "Turn here!" she says, as I overshoot Ashton Reid Road and have to flip a U-turn. She plugs in the iPod and we sing, we laugh, we reminisce about the last time we drove together on the left side while traveling in Scotland, a journey remembered for its rogue sheep, pub lunches, castles and shortbread. Since we travel together so often, we are used to people comparing us. Our commonalities (along with a few differences) are inherited. My fair skin is Irish; Erin's olive complexion is Mexican. We are both insatiably curious about the world; we're both natural hostesses; and we're both lucky to have nice legs (a trait we even share with our brothers). It doesn't take long for us to start comparing the Sister Islands. Both are long and skinny with their heads facing the rising sun and their best dive sites protected on their leeward sides. The Sisters provide refuge for an impressive variety (and quantity) of wildlife ranging from the largest colony of red-footed boobies in the Caribbean, on Little Cayman, to the green parrots that live in a reserve on Cayman Brac. And both are natural beauties, little touched by the glitzy development that now defines their worldly cousin, Grand Cayman. But if Little Cayman is the carefree blonde whose charisma attracts a community of sun-and-fun expats, then Cayman Brac is the brunette with an old soul. Her population of roughly 1,500 is largely descended from the Scottish fishermen who settled along the north shore in the 17th century. Her spine is unbreakable: The limestone bluff for which the island is named ("brac" is Gaelic for bluff) is the outcropping of an underwater mountain range called the Cayman Ridge, which is responsible for all this awesome diving. On the way out to the lighthouse, we pull off to check out Peter's Cave. What we find is a tiny opening in the side of the bluff overlooking the town of Spot Bay. A historic plaque tells us that Peter's Cave has provided the locals with refuge from hurricanes since 1932. Intrigued by the notion of waiting out a terrifying storm inside a cave, we step up and duck through the opening. The first thing we find inside are two plastic lounge chairs with the name Starry written across the top in black ink. But beyond the furnishings, we find a string of chambers tall enough to stand in and whose floors are worn smooth. Back at Divi Tiara, I ask Max the manager if he knows of anyone who has actually used the cave as a hurricane refuge. "That would be Tenson Scott," he says, giving us directions to follow the north road nearly to the end in Spot Bay. "You'll find him in his crafts shop — 'N.I.M. Things,' it's called. He's quite the storyteller, so you might want to form an exit strategy before you go in." Later, we find Tenson, just where Max said he'd be. And indeed, while we ask to see piece after piece of the polished caymanite stone jewelry he's well-known for on the island, he regales us with a string of stories. He tells us how his grandfather took his family, including Tenson's mother, to Peter's Cave in 1932. "And that's why I'm here," he says. He tells us how he's taken his own family up there to sit out six hurricanes. "Those chairs up there are mine," he says, "my wife's name is Starry."

Page [pagebreak] "Hey, what's that?" Erin says, pointing to a necklace that replicates the prehistoric-looking slugs in hard shells that we've seen plastered all over the rocks. She knows perfectly well that she's invited a long story. "That's a chiton," he says, "but we call it sea beef." The story is told at great length while tangents are identified like place markers, which he miraculously returns to.

Eventually we move on, stopping first to photograph the intricate silken spider web outside that dazzles in the late-day sun, and then to eat spicy jerk chicken at La Esperanza. Friday is jerk night on the Brac.

The Challenge We have three days to dive the Brac but only one must-do dive: the MV Capt. Keith Tibbetts, the 330-foot Russian-made destroyer purchased by the Cayman government from Cuba and sunk 10 years ago. After plenty of wall dives — no matter how awesome they've been — a big wreck sounds like a lot of fun. Placid conditions the next morning are reason enough for Divi Tiara to send the boats to Little Cayman where Eagle Ray Roundup and Coconut Walk Wall are welcome additions to our logbook. We finish the day back at the Brac, at a site called Charlie's Reef. Enormous barrel sponges sculpt an Alice in Wonderland set where instead of a white rabbit, mock turtle and Cheshire cat, we see scrawled filefish in their clown makeup, a stranded conch inside a sponge, tank-like lobsters patrolling their territory and tons of schooling fish. Sand chutes are rides that spit us out into the otherworldly blue. We still have two days left to dive the Tibbetts, and we make sure the crew knows our wishes. But Max has another idea. "Dive it from the shore," he suggests. "Hardly anyone ever does it, but I'm telling you, that's my favorite way." And in that moment, the adventure we didn't realize we'd been waiting for takes shape. We know we want to save it for last. We spend the next morning diving the Brac. The East Chute Wreck of the Cayman Mariner is a broken-down barge that's been housing marine life for 20 years. Lime-green pipe sponges splash color across the fractured frame. A French angelfish takes cover. Out in the sand flats, a bar jack rides the curled lip of a stingray's wing, the two performing an act of symbiosis that allows one to hunt in safety and the other to eat the leftovers. Between the wreck and the ledge is a hillock of coral that we explore from base to tip. An iridescent goby hovers over its shell-lined hole in the sand like an angel, and a spotted eel is an apparition that I never see again after turning my attention away to signal Erin. There's so much marine-life gold in this hill that the dive stretches out to an hour. On our way back, Capt. Scott, a cool Canadian from Nelson, British Columbia, gives us an extended briefing to help prepare us for our next day's dive. He slows down directly over the Tibbetts to describe its position in relation to the three mooring buoys. "Swim on the surface to the first, he says, "then begin your descent. The second buoy is over the rear deck which you'll hit at 45 feet." Scott points out the landmark on the shore. "There's a big pool in front of that pile of rocks. Do you see it? Giant stride in there, but watch out for urchins." And finally, he gives us his entire dive briefing, eraser board and all, so we'll be ready for our shore dive the next day.

Last Dive There's a diver-down flag attached to a buoy and reel waiting with our tanks when we stop by the dive shop in the morning. It's nice to know the crew has prepared for us, even though we aren't going out on their boats today, our last on the Brac. We load it all in the trunk of our car and head directly across the island, beyond the airport, and turn down Robert Foster Lane, just as Scott had directed. I pull over on the shady side of the road where we set up our gear and pull on our wetsuits, then walk down to the dredged-out pool. From here, the buoys seem a long way out. Erin cut her teeth as a PADI divemaster doing shore dives in La Jolla, California, so this is child's play to her. But I've got maybe 20 shore dives in all and though I'm stoked that we're here on our own and will have the entire wreck to ourselves, I'm a little reserved. I'm not certain how to gauge the distance or how much energy it will require to reach the dive site. In other words, will it kick my butt before we even get there? But that worrying voice cycling through my mind evaporates the moment we're in the cool water kicking out to the site on our backs, the sun on our faces and seabirds making the only sounds. We take our time, and when we reach the sailboat tied to the first mooring buoy, we spot the next one that marks the stern of the Tibbetts. "Ready, Sister?" Erin shouts out. I give the OK and stick my face in the water for my first look. The viz is as clear as could be and the ship's port side is a wall of iron before me. But what takes my breath away is the spotted eagle ray directly below us, the sun glistening off its white-ringed back as it continues its hunt, cutting lazy circles interrupted only when it burrows into the sand after its prey. We continue down to the deepest point of the bow at 85 feet and work our way up to the antiaircraft guns then penetrate the decks at the fore. Erin rubs her belly when we see an entree-sized scallop, and she points out the purple sponge-like circle of sergeant-major eggs. The eagle ray passes over the deck and disappears around the stern. I'm more curious to follow it than continue my inspection of the wreck. Erin falls in line too as the ray leads us into another world. Before us a field of garden eels fills my view clear to the horizon. I'm reminded of the sea of sunflowers we'd once driven past in Spain, their long necks extended just as the eels are. I look to Erin and spread my arms wide and then cut the air for emphasis. "I've never seen so many in my life," I'm saying. Erin points at herself, "Me neither!" It's finally time for our safety stop, and we swim about halfway back to shore underwater. Back at the car we slip out of our gear and set everything out to dry while we sit out on the rocks to recount the dive, what we saw and how we felt each step of the way. It's peaceful out here and I'm reveling in the confidence boost of this experience, trading in the luxury of the dive boat for this DIY adventure alone with my sister.

Page [pagebreak] She and I sit out on those stones under the sun, watching the chitons cling to the edges of the tidal pools and reminiscing about my first dive trip. She'd challenged me to get certified and join her in Bimini. I remember thinking how lucky I'd be to do my first certified dives with Erin, because only she could know me well enough to read the subtleties of my body language underwater. We talk about her wedding, and she tells me the stories I love about her three-year-old daughter. I share with her my hopes and dreams about my career, my husband and the family I hope to have one day. And in this moment, I realize this isn't the last dive at all. It is only the beginning.

Cayman Airways 800-G-CAYMAN www.caymanairways.com

Cayman Islands Department of Tourism www.divecayman.ky

Nature Cayman www.naturecayman.ky

Sister Islands Tourism Authority www.sisterislands.com

Little Cayman Dive Resorts/Hotels Paradise Divers/Paradise Villas 877-3-CAYMAN www.paradisevillas.com

Pirates Point Resort 345- 948-1010 www.piratespointresort.com

Reef Divers at Little Cayman Beach Resort 800-594-6703 www.littlecayman.com

Southern Cross Club 800-899-2582 www.southerncrossclub.com

Cayman Brac Dive Resorts/Hotels Brac Caribbean Beach Village and Carib Sands 866-THE-BRAC www.braccaribbean.com

Reef Divers/Brac Reef Beach Resort 800-327-3835 www.bracreef.com

Divi Tiara Beach Resort 800-367-3484 www.divitiara.com

Live-aboards

Cayman Aggressor IV 800-348-2628 www.aggressor.com