More than two dozen earthquakes in 48 hours have shaken Mount Spurr, alarming scientists and locals.
Experts say rising magma, increased gas emissions, and grey vapour suggests an eruption could happen within weeks or months.
If it erupts, Mount Spurr could spew ash 50,000 feet into the sky, disrupting global air travel and blanketing Anchorage in dust.
The last eruption in 1992 caused major airport closures and millions in damages – residents are now bracing again.
The next warning sign scientists are watching for: a sustained volcanic tremor, signalling magma pushing toward the surface.