Just two words from the dive guide gets our pulses racing. “Whale shark!” Instantly, everyone is crowding to one side of the boat, hoping for a glimpse. We fumble with masks, snorkels and fins. This is really going to happen. We jump ahead of the shark and hang in the water, waiting for it to materialize. It doesn’t disappoint. Its mouth is as wide as I am long, and it powers through the water with unstoppable momentum. It hardly seems to be trying, lazily wafting its tail, which still propels it faster than we can swim. My brain tells me this giant feeds only on small fish and plankton, but swimming with a shark longer than the boat you’ve just jumped off gets the heart pumping.

Whale sharks are the biggest fish in the sea. The longest on record is 60 feet, but they likely get even bigger. The Maldives, the Seychelles, Cenderawasih Bay in Indonesia, Isla Mujeres and La Paz in Mexico, Ningaloo Reef in Australia, Galapagos and Mozambique are a few locales famed for reliable encounters, though it’s rare to see the biggest ones—most sightings are with young males, which can still be an impressive 30 feet long.

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The second biggest fish, the basking shark, is even more exclusive. Recent studies suggest that worldwide, there might be fewer than 10,000 individuals—though some estimates suggest it may be double that. Encounters with this scarce and secretive cold-water specialist are less common. It is blotchy gray, classically shark-shaped and swims with its cavernous mouth agape as it sieves plankton from the water. It may be second in size, but for me, it’s a bigger prize.

The British Isles are the current hotspot, although the best location to see them does seem to shift every decade. Twenty years ago, prime sightings were off the tip of southwest England; then, about 10 years ago, they began to appear more around the islands along the west coast of Scotland. Today, you’ll have the best chance of spotting basking sharks off the west coast of central and northern Ireland. Even so, finding them depends on cooperative summer weather that brings both plankton and sharks to the surface, something that isn’t exactly reliable in these locations.

Both species undertake huge migrations in pursuit of the best feeding conditions, and we’re only just filling in the details. Basking sharks tagged in the northern Atlantic have been known to cross the equator, and one whale shark traveled a whopping 8,100 miles in just three years. The baskers come to Western Europe in early summer to catch the explosion of zooplankton that follows the spring bloom, while whale sharks appear at seasonal plankton blooms and the spawning events of corals, fish and even the red land crabs of Christmas Island.

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Capturing quality images requires both stealth and respect. Neither can be chased down, and the ideal approach is to be quiet in the water and let the shark come to you. Almost all encounters are when snorkeling, and there is no need to use strobes, which will limit your mobility. Watch the sharks from the boat for a while to understand how they are moving, then get dropped into the water well ahead of the shark, float up to the surface and avoid splashing. If you disturb basking sharks, they will simply close their mouths, and you won’t get that iconic shot.

While I am waiting, I will wipe my hand across my dome or lens to eliminate any small bubbles, and then I focus on my fin. When a shark is close, autofocus works well, but when it first appears through the murk, autofocus is easily confused. I’ll take a shot to check exposure, ensuring I get a well-exposed water color. If the blue is right without the shark, it will be right with the shark. To light up the subject, swim just slightly toward the sun when the shark appears—this is enough to make the shark alter its course slightly, with the sun lighting it up perfectly.