Over the next four decades, aerial images documented the steady, eastward retreat of both the Alsek Glacier and another ice mass to the south called the Grand Plateau Glacier. As both glaciers melted, Alsek Lake grew significantly, filling in the void left by the Grand Plateau Glacier and gradually overtaking the ice that once surrounded Prow Knob.

This summer, NASA’s Landsat 9 satellite observed the last bit of ice pulling away from Prow Knob, officially turning the landmass into an island.

The space agency estimated that the separation occurred sometime between July 13 and Aug. 6.

In the early 20th century, the Alsek Glacier was far more extensive, reaching all the way to a landmass known as Gateway Knob, located roughly 3 miles west of Prow Knob, according to glaciologists. Since 1984, the glacier has retreated more than 3 miles, according to NASA.

Alsek Lake, in turn, has nearly doubled in size since 1984, growing from around 17 square miles to almost 30 square miles. The runoff of meltwater into the glacier basin forms what is known as a “proglacial lake.”

ditional melting is expected in the coming years, NASA said, particularly now that the ice has detached from Prow Knob and is thus less stable.