Extreme weather at Virginia’s Washington Dulles International Airport forced passengers to take shelter underground on Monday.

Fairfax County, Virginia – where the airport is on the border – issued a tornado warning as severe storms push through the state. The tornado warning expired later Monday.

The airport released a statement on Twitter warning passengers to “stay away from windows and listen to safety instructions provided by airport operations and gate agents.”

Officials later said that the runways reopened and airport operations were resuming, but that the wicked weather could trigger more flight delays.

The airport also apparently moved fliers underground to the train tunnel out of an abundance of caution.

“Tornado warning at Dulles airport. All passengers sent underground to the train tunnel,” American Enterprise Institute president Arthur Brooks tweeted.