The Hoover dam experienced a large explosion on Tuesday as a tour of the site was taking place, with a video of the incident quickly circulating online.

The clip shows a section of the facility ablaze with a massive black plume of smoke shooting upward.

People touring the dam when the accident occurred reported hearing a loud explosion and seeing fire and smoke.

The incident is believed to have been caused by a faulty transformer or generator. No official statement on the cause of the explosion has been released.

A tour guide giving guests a tour of the 726-foot concrete structure suggested that people record the fire on their phones, the New York Post reported.

“There’s just been an incident here at the dam,” the guide said. “We’re going to be leaving now so we don’t get trapped in here.”

The fire was put out without further incident, according to the City of Boulder’s Twitter account.

“The fire was extinguished before Boulder City Fire Department arrived on scene. Bureau of Reclamation/Hoover Dam will be handling any additional questions,” the city said.

Hoover Dam spokesperson Michelle Harris told the Post: “There is no risk to the power grid and power is still being generated from the powerhouse.”

The cause of the blaze is under investigation.

The Hoover dam is located on the border between Nevada and Arizona, around 30 miles from Las Vegas, and opened in 1936. It is the world’s most popular dam, with 7 million tourists visiting every year, the National Park Service said on its website.