GREENLAND — This summer, a chunk of ice the size of lower Manhattan broke off of a glacier in Eastern Greenland. It contained 10 billion tons of ice, making the video of the event an insanely shareable capsule of climate change dread. But for NASA scientists, the spectacle created by these massive calving events is really just the final step in a far more worrisome — and less visible — process.

That's because glacial melt isn’t just the result of our planet’s warming air. The biggest threat to these glaciers’ continued existence resides deep below the water’s surface.

Unlike most other bodies of water, the ocean surrounding Greenland gets warmer with increasing depth. That’s because warm, salty currents from the Atlantic are heavier than fresh glacial water, so those currents end up on the bottom. And that’s what’s got scientists’ attention: our oceans absorb the heat trapped by greenhouse gases, so they’re getting warmer, and as they do, Greenland’s biggest, deepest glaciers are interacting with them — and melting at increasing speeds.

To understand this better, NASA has been sending planes and boats to Greenland in an effort to map the ocean floor. What they're seeing isn't good.

“We'll have to revise our sea level projections upward, and that's scary,” said NASA climate scientist Josh Willis, who cautioned that they're still in the early days of the ocean mapping mission, dubbed “Oceans Melting Greenland” — or "OMG". "If we're reshaping the coastline in a radical way, you know do you want to take out a 30-year mortgage on a house that might be flooded in 30 years? And so it's real and it's time to start dealing with it."

VICE News Tonight travelled to Greenland to visit Eric Rignot, a NASA researcher who sails the iceberg-infested waters in the hopes of figuring out how fast Greenland’s glaciers are melting — and how much trouble we’re really in.

This segment originally aired Octoer 3, 2018 on VICE News Tonight on HBO.