America’s cities and scenery may be spectacular, but have you ever thought about what lies beneath? From labyrinthine caves and hidden towns to subterranean hot springs and even a zipline park, these cool attractions are all tucked away.
Read on to discover the most awe-inspiring underground experiences in the USA...
Hidden beneath the West Virginia wing of the Greenbrier resort, behind 18-tonne blast-proof doors, is the Greenbrier bunker. It was built as a top-secret project in the late 1960s, to safeguard the US Senate in the event of nuclear war. Designed to house 1,100 people, the two-storey building has bedrooms kitted out with bunk beds, decontamination showers and its own power plant. Tours are available.
Delve beneath the buzzing cafés of City Market in Indianapolis and you'll find an eerily quiet network of catacombs, stretching for 22,000 square feet (2,044sqm). The limestone and brick arches and pillars were originally the basement of Tomlinson Hall, a vast municipal building that was demolished after a fire in 1958. An organised tour gives visitors the chance to explore the musty underground warren, which is popular with ghost hunters. However, the tours are currently suspended due to construction work around the City Market.
Deep in the belly of Onondaga Cave State Park sits Onondaga Cave, a subterranean world of spectacular stalactites and lofty stalagmites. The cave system includes the Lily Pad room, so-named for its lily-like calcite deposits, resting gently on a crystal clear pool. You can take a tour, but bring a jacket as it can get quite chilly no matter what time of year.
The San Luis Obispo County stretch of the Californian coast is home to a world of grottoes, arches, rock gardens and caves. It's possible to explore their watery depths on a kayaking excursion – some of the caves run up to 30 feet (9m) deep. There are seals, otters and dolphins around too.
Love this? Follow our Facebook page for more travel inspiration
In 1861, Sacramento was hit by 45 days of powerful Pacific storms, which caused the rivers to surge through the town, sweeping thousands to their deaths. Instead of deserting their hometown, locals spent 15 years slowly raising the streets and houses by nine feet (2.7m). Today the Sacramento History Museum runs tours around some of the abandoned basements, alleyways and underground streets.
The Texas Highway Department stumbled across what's now known as Inner Space Cavern in 1963, when drilling through rock to test if it could support an overpass. When a drill fell through the roof of the Discovery Cave, the workers realised they were above a vast subterranean network of chambers, with sharp stalagmites, still pools and even coral (this was once the ocean floor).
The Binkley Cave system opened to the public in 2013 as Indiana Caverns and holds the title of the longest cave in Indiana – it's also in the top 10 longest caves in America. Activities on offer include cave tours and kayaking on a subterranean river (look out for the cave crayfish).
Tucked beneath overhanging cliffs in Mesa Verde National Park are around 600 sandstone cliff dwellings, dating back some 820 years. These ancient villages were built by Ancestral Pueblo Indians who farmed the land above. You can access the unique structures in the Wetherill Mesa part of the park via self-guided or ranger-led tours.
This rusty red cavern is only four feet (1.2m) wide and deep, but its vibrant colour and otherworldly rock formations certainly pack a punch. Tucked away down a dirt track in Nevada’s Valley of Fire State Park, the sandstone rock has slowly been eroded by wind, forming the Windstone Arch and alien-like shapes. If you want to see it for yourself, check the GPS co-ordinates as it isn’t easy to find.
Discover these charming small towns that hide amazing underworlds
When a fire razed Seattle to the ground in 1889, the city was rebuilt one storey higher to lift it out of the boggy land. Roads were filled in and raised, and buildings reconstructed so their ground floors became basements. New pavements then bridged the gap between the raised roads and first floor entrances, leaving tunnels below. Visitors can explore these subterranean passageways on a guided tour, taking in shop facades, bank vaults and even a toilet.
If you loathe lifts, you might not relish the 90-second descent into Strataca, an underground salt mine museum – but it’s worth the ride. The lift takes visitors 640 feet (195m) down into the vast mined-out area. Here, you’ll see giant salt rocks, learn about mining through the ages and take a train ride through areas that were actively mined in the 1950s. A tram will take you deeper, if you dare, even turning off the lights for a few seconds so you can experience true darkness.
Buried on the fringes of Shenandoah National Park, Luray Caverns is hard to beat for its staggering scale. You’ll find epic chambers with ceilings over 10 storeys high, as well as crystalline pools such as Dream Lake, which has a surface so glassy it perfectly mirrors the stalactites above. There’s also the legendary Great Stalacpipe Organ, which can play Beethoven's Moonlight Sonata by using rubber mallets to strike 37 stalactites of varying sizes.
With its rolling waves of vibrant red rock, Antelope Canyon looks like it could belong on Mars. Eerie beams of daylight pierce through from above and cast shadows on the dusty canyon floor. The sandstone walls, which tower up to 100 feet (30.5m) high, have been sculpted into their wave-like shapes by the wind over millions of years. The canyon can only be visited on a guided tour as it's located within the Navajo Nation.
Check out these stunning images of the world's most beautiful canyons
What do you do with the basement that housed Los Angeles’ first power plant? You turn it into a bar of course, keeping the turbine steam generators and utility tunnels as a feature. Enter The Edison, one of the hottest spots in downtown LA, legendary for its cocktails and fizzing burlesque shows.
Over 119 caves make up New Mexico’s Carlsbad Caverns. Within this vast cave network, 750 feet (229m) deep underground, is the so-called Big Room, a chamber so huge it could hold over six football pitches. The caves are home to half a million bats that can be seen swirling out into the night sky at dusk. Advance reservations are required to enter the national park.
It took nearly 15 painstaking years for workers to blast this 2,000-foot (610m) man-made cave out of solid rock. Created in the 1850s underneath the city of Lockport in New York, the chamber was designed to supply water from the Erie Canal to local industries. Underground boat tours can be booked in advance.
Take a look at the strangest sights in America's biggest cities
Hiding beneath Butte, Montana is a deserted secret world, including a barber, shoe shops, a prison and the Rookwood Speakeasy, a hidden bar from Prohibition days. These are well-preserved relics from the early 20th century, when mining was at its height and the population swelled to around 100,000. Space was at a premium so people started using underground areas connected by tunnels. Today, you can visit on a subterranean tour.
Over millions of years, subterranean streams dissolved the limestone rock to form this spectacular cave system in Nevada's Great Basin National Park. During Prohibition it was used as a secret speakeasy but now the attraction is the thousands of delicate columns, hollow stalactites (known as soda straws) and needle-like crystals. Look out for "cave popcorn", formed from small lumps of calcite. It looks surprisingly like the real thing. It's recommended that you book a ranger-led cave tour in advance as they regularly sell out.
This geothermal spring in Utah is buried within a dome-shaped limestone rock. Here, you can swim, snorkel and scuba dive in the blue, bath-like water (it’s 35°C/95°F). For divers, the charm isn’t the marine life (there is none), but the experience of diving down to 45 feet (13.7m) without the need for a wetsuit. Reservations need to be made in advance for the self-guided tour.
In the 19th century, hopeful miners explored this cavern in Branson, Missouri on a quest for silver. Instead, they discovered bat dung so they set up a lucrative operation, selling it for fertiliser. Down some 600 steps, there are waterfalls and the epic Cathedral Room, big enough to house the Statue of Liberty. Marvel Cave is part of the theme park Silver Dollar City.
Deep under the South Dakota grasslands is Wind Cave, one of the world’s longest cave networks. Here, the oldest rock formations date back some 310 million years and powerful winds blast in and out of the cavern mouth, causing a whistling noise. To date, over 123 miles (198km) of rocky passages have been explored and six lakes discovered. Here the big draw is boxwork, where calcite forms in an intricate honeycomb pattern.
Louisville Mega Cavern is actually a vast 100-acre limestone quarry, hollowed out by blasting when it was mined in the mid-20th century. Today, it houses Mega Zips, which boasts the title of the world’s only fully underground zip line course. Get your blood pumping as you slide through the air, reaching heights of up to 70 feet (21m).
Discover the world's most incredible underground attractions
The unspoiled Black Canyon is just a 45-minute drive from the bright lights of Las Vegas. Emerald Cave is the highlight on the Black Canyon National Water Trail, with its vivid green sparkling water. Visitors can kayak through the still waters of the cove, then paddle out of the glare of the sun and into the caves that burrow into the cliff face.
With its leaded skylights, vaulted tile ceiling and chandeliers, the abandoned City Hall subway station makes you wonder why other stations have to look so functional. City Hall opened in 1904 but closed in 1945 due to short platforms. The New York Transit Museum runs regular tours of the station, but you must book in advance.
Take a look at more amazing abandoned subway stations around the world
Called mammoth for a reason, this labyrinthine network of caves within Mammoth Cave National Park has more than 405 miles (652km) of mapped passages, making it the longest system in the world. And now a team of researchers has discovered eight more previously unknown miles (13km) of underground passages. Visitors can explore 10 miles (16km) of tunnels on a guided hike. The formations are staggering – don't miss Frozen Niagara which looks just like a waterfall sculpted out of rock.
Ruby Falls is hidden 1,120 feet (341m) inside Tennessee’s Lookout Mountain. This cascade of water tumbles 145 feet (44m) into a limestone cave, against the backdrop of music and illuminated by colourful lights. Guided tours take you on the underground walk to the falls and, if bright lights and music aren’t your thing, there are limited spots available on evening tours, where the walk and falls are lit only by lanterns.