A crater six feet wide and three feet deep appeared overnight near the end of a runway at Rome’s airport, spewing clouds of toxic gas 15 feet into the air.

The crater was discovered by drivers Saturday morning in the middle of a roundabout 900 feet from a runway’s end. Crowds had gathered to look at the crater until firefighters and vulcanologists closed the area because of deadly gas believed to be a mix of carbon dioxide, hydrogen sulfide and methane.

Tests are underway to determine the chemical makeup of the gas.

Officials are trying to claim the gas was simply rotting organic matter trapped underground but a seismologist told the Daily Telegraph a volcanic event was more likely the source.

“Gas underground can remain hot for tens of thousands of years after volcanoes erupt,” Alberto Basili said. “Every now and then it can rise to the surface from miles underground.”

“We have seen things like this elsewhere around Rome, with farm animals being killed after they breathe in the gas.”

Rome’s airport is the 6th largest in Europe with 37 million passengers a year. Operations of the airport are expected to not be impacted unless a further incident takes place.