This frozen tunnel in central Alaska is both an engineering feat and a valuable climate classroom.

The world's longest and deepest train tunnel.

A historic observatory precariously perched at the "Top of Europe."

The abandoned funicular tunnel was a secret base for the BBC during World War II.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

Hidden beneath the streets of London is a secret subway that only carried mail.

The world’s largest concentrated solar plant generates as many problems as megawatts.

The deepest metro station in the world.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

This underground power plant looks more like a super-villian's lair than a public work.

An abandoned network of military bunkers and tunnels open for anyone willing to descend into the darkness.

New fungal and bacterial species call this deadly lake home.

The longest navigable underground river in the United States is home to countless blind and albino animals.