Experts say a volcano responsible for causing one of the largest disasters in the Western Hemisphere is showing signs of increased activity, which has forced the Colombian government to issue evacuations in the potential impact zone.

The Nevado del Ruiz volcano sits about 80 miles west of Bogotá, and according to Colombia’s Geological Survey, an earthquake swarm has increased to levels not seen since a disastrous eruption in the 1980s killed at least 25,000 people.

Authorities have raised the alert level to orange – the second-highest on a four-stage scale.