NASA experts have discovered a huge cavity that grew at the bottom of Thwaites Glacier in West Antarctica.
NASA, described by NASA as "alarming", is two-thirds of Manhattan and almost 300 meters high.
This huge gap is enough to contain 14 billion tonnes of ice, most of which melted over the last three years.
The co-author of the study, Eric Rignot, said: "For years, we have thought that Thwaites is not firmly attached to the base underneath it.
"Thanks to the new generation of satellites, we can finally see the details."
Researchers discovered a cavity using ice-penetrating radar as well as space radar data.
Pietro Milillo, who led the study, said: "[The size of] Coal beneath the glacier plays an important role in melting. As the glacier gets more heat and water, it melts faster. "
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Thwaites Glacier itself is about Florida in size, and is currently responsible for about 4% of global sea level growth.
Worryingly, when the glacier collapses, the globe would raise the world's oceans by more than two feet.
Its collapse could also destabilize neighboring glaciers, which would increase sea levels by another eight feet.
The results show that the interaction of the ice ocean is more complex than previously understood.
Milillo added: "Understanding how the ocean melts, this glacier is essential for predicting its impact on sea level rise over the coming decades."