On our first morning at the Thwaites Glacier in West Antarctica, the air was eerily still. The captain of our research icebreaker, encouraged by the calm, made a bold choice: Our ship would hold close to the ice shelf so that the sonar system would peer a little ways beneath it while generating a detailed map of the seafloor. The scientists on board, along with the writers like me, were the first people in the history of the planet to visit this part of Thwaites. Our task was to bring back as much information as possible about the place where ocean and ice meet.

The mood on the ship shifted into overdrive. Sleep took a back seat to data collection as autonomous vehicles surveyed the troughs where relatively warm water pushes beneath the ice, eating away at it from below. Coring devices carried back sediment the glacier spat out the last time it retreated, and even several Weddell seals were outfitted with transponders that recorded the temperature and conductivity of the water all around the unstable glacier. Every single bit of information that came on board taught us something about Thwaites’s past and present, which would help scientists to better predict its future.

If Antarctica is going to lose a lot of ice this century, it will likely come from Thwaites. If it disintegrated, it would be responsible for over two feet of sea level rise, and its collapse could destabilize the entire West Antarctic Ice Sheet, causing global sea levels to jump 10 feet or more. In terms of the fate of our coastal communities, this particular glacier is the biggest wild card, the largest known unknown, the pile of coins that could tip the scales one way or another. Will Miami even exist in 100 years? Thwaites will decide.