As a freelance underwater photographer, I’m accustomed to exploring exotic and sometimes unusual dive sites. But when I got the call to join a professional diver to try subway diving, I figured this would be among the strangest dive site of all.

I learned that our actual dive site was a rusty little manhole behind the Scheidplatz subway station in Munich, Germany. “When subway tunnels are built, they often cut through groundwater streams and sometimes block them,” said my dive buddy, Max. “Large pipes and ducts have to be built to allow the groundwater to flow into 60-foot-deep shafts.” My mission was to photograph Max as he dived into one of these “collector” shafts for inspection and maintenance.

The first challenge was squeezing through the narrow manhole in my bulky drysuit. (January in Munich is brutally cold, to the tune of 14 degrees F on this day.) After climbing down a ladder, I sat on a little steel platform above the water, with barely enough space to put on my tank and prepare my camera. Then I descended 60 feet through crystal-clear water to the bottom of the shaft. There, Max inserted a robotic camera into one of the large pipes, which allowed the engineer sitting in the heated truck above us to comfortably scrutinize the inside of the pipe system.

Down here in the cold, dark shaft, surrounded on all sides by concrete walls, I realized the most important qualification for subway diving was to not be claustrophobic — especially when the water turned completely dark as Max began removing sediment from a pipe. With zero visibility, taking photos was impossible; after 30 minutes in the shaft, I fumbled around to find the ladder before slowly making my way back up to a cup of hot chocolate.