A dozen people and a dog were evacuated from an observatory near Mount St. Helens by helicopter after a landslide blocked a nearby highway.

The Washington State Department of Transportation said Monday that State Route 504 near Mount St. Helens was closed in both directions after a landslide Sunday night.

The King County Sheriff’s Air Support Unit said on Twitter around 11 a.m. Monday that the unit had just finished evacuations from the Johnston Ridge Observatory near the volcano.

MONTANA HIKER, 19, FOUND ALIVE DAYS AFTER GOING MISSING IN GLACIER NATIONAL PARK: OFFICIALS

CLICK HERE TO GET THE FOX NEWS APP

State transportation officials said Monday afternoon that the road remained closed in both directions at the seasonal gate near milepost 45 and that drivers should plan for an extended closure.

There may be significant structural damage to the roadway, officials said.

On May 18, 1980, the Mount St. Helens volcano in Washington state exploded, leaving 57 people dead or missing. Mount St. Helens National Volcanic Monument since then has become a world-class outdoor laboratory for the study of volcanoes, ecosystems and forestry, in addition to a recreational and tourist destination.