Even if you've been to the Catacombs of Paris or have gone spelunking in Brooklyn's abandoned subway lines, there's still a wealth of underground places to be explored. Here are some of the world's less welcoming manmade tunnels and grottos.

Fort Zverv
In the 1970s, this munitions store in Kronstadt, Russia caught on fire. The heat and incendiaries going off within the enclosed fort warped its brick walls, transforming its rooms into haunting stalactite-like caverns. You can see more photos of its netherworldly ruin here.


In addition to the structures above, we have oodles of stories about tunnels, storehouses, creepy corridors, and life below the surface in the io9 archives. Here's a bevy of real (and a handful of speculative) subterranean spots found around our planet:

-The subterranean sprawl of New York City's Freedom Tunnel
- Project Iceworm: the nuclear city hidden under Greenland's glaciers
- In an alternate universe, there's a 600-mph underground missile train between NYC and Philly
- The secret cave city under Nottingham
- The industrial infinity of Kiev's metro tunnels
- A subway spelunker's guide to Paris' abandoned Métro stations
- A 1934 Map Of The Secret Lizard City Under Los Angeles
- The City-Sized Nuclear Bunker Chairman Mao Built
- Diving Into the Wreck of Copenhagen's Metro System
- Ride Out 2012 In Style With Vivos Apocalypse Shelters
- How to turn a highway into a giant dungeon (slightly NSFW)
- Live In An Underground Hobbit Bunker
- In Bosnia, Tito's secret bunker is revealed
- Plans for a domed city in a kilometer-wide Siberian diamond mine

The Củ Chi Tunnels of Vietnam
During the Vietnam War, the Vietcong built approximately 150 miles of cramped tunnels under the Ho Chi Minh City suburb of Củ Chi. The tunnels were used by North Vietnamese soldiers to avoid bombing campaigns and evade US troops, but they also grew to accommodate a civilian population as villages were bombed on the surface.


Life underground was almost as miserable as life topside — spiders, snakes, intestinal parasites, claustrophobia, and remembering where you rigged the booby traps and spike pits were part and parcel of tunnel life. You can watch a documentary on life in the Củ Chi Tunnels here.

Željava Air Base
The world's largest underground airport was built in 1965 in present-day Bosnia-Herzegovina. The Yugoslavian government constructed this massively expensive facility to house 1000 military officials and to withstand a nuclear strike on par with Nagasaki. The facility was destroyed in conflicts stemming from the dissolution of Yugoslavia in the early 1990s and is currently a precarious place to visit given the amount of unexploded mines in its environs.

Portland's "Shanghai Tunnels"
Underneath the streets of Portland are a series of subterranean passages that lead to the Williamette River. These tunnels were built in the 19th century to transport goods to the city's port, but according to popular lore, these passages were used for a more sinister purpose: abducting unsuspecting Stumptowners and "Shanghaiing" them on to trade vessels headed across the Pacific. Even though this form of black market impressment did occur in Portland, modern scholars doubt that the tunnels were the epicenter of such maritime kidnappings.

The Okinawa Naval Tunnels
During World War II, the Japanese military built a underground naval facility under a hill near the town of Tamagusuku. On June 13, 1945, admiral Minoru Ōta and his forces were faced with defeat at the hands of the American military. Rather than surrender, Ōta and his troops committed suicide in the complex. You can still see the shrapnel scars where the soldiers set off their grenades.