Scientists in Antarctica have recorded, for the first time, unusually warm water beneath a glacier the size of Florida that is already melting and contributing to a rise in sea levels.

The researchers, working on the Thwaites Glacier, recorded water temperatures at the base of the ice of more than 2 degrees Celsius, or 3.6 degrees Fahrenheit, above the normal freezing point. Critically, the measurements were taken at the glacier’s grounding line, the area where it transitions from resting wholly on bedrock to spreading out on the sea as ice shelves.

It is unclear how fast the glacier is deteriorating: Studies have forecast its total collapse in a century and also in a few decades. The presence of warm water in the grounding line may support estimates at the faster range.

That is significant because the Thwaites, along with the Pine Island Glacier and a number of smaller glaciers, acts as a brake on part of the much larger West Antarctic Ice Sheet. Together, the two bigger glaciers are currently holding back ice that, if melted, would raise the world’s oceans by more than a meter, or about four feet, over centuries, an amount that would put many coastal cities underwater.