Oh, dam!

A large explosion rocked the Hoover Dam during a tour of the facility early Tuesday, dramatic video shows.

A clip posted to Twitter minutes after 10 a.m. local time showed a huge plume of smoke emanating from the dam along the Colorado River on the border of Nevada and Arizona.

“Touring the #hooverdam and heard an explosion #fire,” the visitor, Kristy Hairston, tweeted.

An official giving a tour of the 726-foot concrete structure told visitors to get their cellphones and cameras out to catch footage of the blast.

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

It was not immediately clear what led to the explosion, although some on social media speculated one of the facility’s generators may have caught fire and blown up.

Firefighters from Boulder City were en route to the dam after getting an emergency call, department officials in Nevada announced.

But the blaze was put out before Boulder City firefighters got there, the department said in a subsequent tweet.

A transformer at the dam caught fire and was extinguished by the facility’s fire brigade at about 10:30 a.m., according to a spokeswoman from the Bureau of Reclamation, which manages the facility.

“There is no risk to the power grid and power is still being generated from the powerhouse,” spokeswoman Michelle Harris wrote The Post in an email.

The exact cause of the fire remains under investigation, Harris said.

The Hoover Dam — about 30 miles southeast of Las Vegas — is the most-visited dam worldwide, drawing about 7 million tourists annually, according to its National Park Service website.

Some 5 million barrels of cement and 45 million pounds of reinforced steel were used to build the dam, a five-year undertaking completed in 1936.