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The Muldrow Glacier at Denali National Park and Preserve in Alaska began “surging” in March for the first time in 64 years, according to the National Park Service (NPS).

A surge happens when ice within a glacier “advances suddenly and substantially,” according to the NPS. About one percent of the world’s glaciers surge, the NPS said.

The NPS said scientists were anticipating the event, as the Muldrow Glacier has a history of surging about every 50 years. The park said scientists “are now racing to document this rare geologic event before it ends” by monitoring and tracking the glacier using satellite data and conducting aerial surveys.

Signs of the “long-awaited” Muldrow Glacier surge were first spotted on March 4 by a local pilot. The NPS said the glacier is “usually slow moving and relatively smooth,” but the pilot noticed new cracks throughout the ice.

The Muldrow Glacier is a 39-mile-long glacier that is found along Denali, the highest mountain peak in North America. The NPS said the surge is causing the Muldrow Glacier’s ice to move more than 100 times faster than normal.

Denali National Park and Preserve released this footage on April 7 that shows crevasses along the surging Muldrow Glacier. The park said the footage was filmed on March 28. Credit: NPS Video via Storyful