Underground Facts

An underground fire in Australia has been burning for approximately 6,000 years, making it the world's oldest known coal fire.

Mexico City is built on top of an underground resevoir!

In 1963, a man found a mysterious room behind a wall in his home that lead to a huge underground city that once housed up to 20,000 people, complete with schools, livestock pens, and food stores, dating back over a millennium.

A mole can dig a tunnel 300 feet long in just one night!

A mole can dig a hole 300 feet deep in one night.

The largest underground hotel suite in the world is located 21 stories below the surface in Arizona's Grand Canyon Caverns. The Cavern Suite sits in a 345-million-year-old cave and costs $1,000 per night.

Mammoth Cave in Kentucky is the longest cave system on Earth - over 400 miles of tunnels. In 1842, Dr. John Croghan moved 16 sick patients underground, convinced that cave air would cure their illness. At least five died inside. Nearly all the others died within weeks of leaving. The doctor died of the same disease in 1849.

The Colosseum has a hidden underground corridor that Roman emperors used to enter the arena unseen and avoid assassination. Called the Passage of Commodus, it sat sealed beneath Rome for nearly 2,000 years. On October 27, 2025, it opened to the public for the first time after a year-long restoration. Part of the tunnel still leads toward an unknown destination.

The U.S. Capitol has run its own private underground subway since March 1909. The original cars were lemon-yellow Studebakers that held eight senators and took just 45 seconds to cover a 760-foot tunnel. It still runs today - and the public cannot get on without a congressional escort.