NASA satellite imagery has revealed a new island off the coast of Alaska, formed when long-standing glacial ice melted, isolating a small mountain that was once part of the mainland.

The island is located within Alsek Lake, where the Alsek Glacier has been gradually thinning and flooding the area with meltwater.

According to a NASA statement, two Landsat images -- captured on July 5, 1984 by the Thematic Mapper (TM) on Landsat 5 and on August 6, 2025 by the Operational Land Imager-2 (OLI-2) on Landsat 9 -- show the transformation in striking detail.

Alsek Glacier once engulfed the small mountain known as Prow Knob. Over the last four decades, both arms of the glacier have receded more than five kilometres, forming a proglacial lake. The most recent imagery confirms that the glacier has completely separated from Prow Knob, which is surrounded by water and officially classified as an island, NASA said.

For decades, glaciologists have monitored the Alsek Glacier. The glacier is thought to have ended in the early 20th century at Gateway Knob, approximately five kilometres west of Prow Knob on the opposite side of what is now Alsek Lake. Since then, the glacier has been steadily retreating, with satellite data documenting the process.

The retreat has fuelled a significant expansion of Alsek Lake, which has grown from approximately 45 square kilometers in 1984 to around 75 square kilometres today. Its expansion is fuelled not only by Alsek Glacier meltwater, but also by nearby proglacial lakes like Harlequin and Grand Plateau. Until around 1999, Alsek Glacier was connected to the Grand Plateau Glacier's northern arm. As both ice masses receded further, they formed a major branch of Alsek Lake, which was visible in later satellite images.

The newly formed island is approximately 5 square kilometres. Based on satellite data, scientists believe it emerged between July 13 and August 6. According to NASA, the separation of Prow Knob has made the glacier less stable and more prone to calving, which occurs when large sections of ice break away into the lake.