For some, just entering the water is enough to trigger the fight-or-flight response. But for those who crave challenges and the thrill of the eerie in a foreign environment, there's nothing that rattles the nerves more than these unusual underwater encounters.
1. Human skull-filled caves
Where: Yucatán Peninsula, Mexico
Why: Cave diving meets archaeology
Danger factor: 4
The skulls and bones of 125 people lie within Las Calaveras cenote, and nobody knows why. The ancient Mayans believed this underworld linked us to the spirits, thus these water-filled caves became sites for rituals and sacrifices to appease the gods. The hardest part of reaching this site is squeezing through the three-foot-square opening leading in, then contending with possible claustrophobia within the overhead environment.
Speaking of claustrophobia, check out these tight squeezes in underground caves.
2. Black-water diving
Where: Kona, Hawaii
Why: The world's most bizarre animal light show
Danger factor: 9
They call it black-water diving. Off the Kona coast, tour operator Jack's Diving Locker brings guests to the edge of the abyss just as night falls. Tethered to the boat, divers drop into darkness to a depth of 50 feet. There, they're met by thousands of animals, each no bigger than a pinky nail and glowing from within, that rise from 5,000 feet below.
Called bioluminescence, this phenomenon lights these tiny critters from within, creating what is easily the most surreal underwater experience that doesn't involve drugs. But if you feel like you're being watched, get out of the water. This is oceanic whitetip shark territory, and dusk is feeding time.
3. An active volcano
Where: Mahangetan island, Indonesia
Why: The mesmerizing appeal of lava and sulfur bubbles
Danger factor: 5
It's not the lava that will burn your skin here, but the sulfuric bubbles rising from the submerged Banua Wuhu volcano, about 16 feet under the surface of the Celebes Sea. Trap them and bring them to the surface, and the smell will burn your nose hair. But when the bubbles escape naturally from this 1,300-foot-tall volcano, the odor simply burns off.
4. A super-creepy underwater cemetery
Where: Lake Jocassee, South Carolina
Why: A thrill appreciated by horror movie buffs
Danger factor: 0
If you want to dive in a cemetery and meet ghosts from the Civil War, Lake Jocassee is the spot. Duke Power electric company flooded the site in 1973 to create a hydroelectric dam, placing this graveyard at a depth of 118 feet. This same collection of tombstones, before submerged, appeared in the movie "Deliverance."
5. Titan missile silo
Where: Royal City, Washington
Why: Slip inside a long-lost era
Danger factor: 6
Algae encrusts the "Danger: High Voltage" signs near the entry. Drop about 110 feet into the launch tubes and emergency hatches of this Cold War relic, now a flooded warren that once held missiles, control centers and living quarters for those who manned this station. The structures still contain rusted beams and hanging wires that may cause entanglement, but the biggest danger is light failure — thus every diver venturing inside must carry at least two.
6. Weapon of mass destruction
Where: Madison Aquatic Park, Alabama
Why: Dr. Strangelove photo ops
Danger factor: 2
The site itself — a lake in the middle of rock quarry — is nondescript. That is, until you see the nose of the 52-foot-tall 1962 NASA Minuteman missile shooting straight out of the water. For the full effect, drop to the bottom and peer up at this Cold War souvenir — you'll feel chills.
7. Turtle tomb
Where: Sipadan Island, Malaysia
Why: It's a natural cemetery
Danger factor: 6
No, the piles of turtle bones don't serve as evidence of any hierarchical rankings or sacrifices in the sea turtle community. Rather, it's more likely the turtles, as well as a few dolphins, found their way inside this cave and got lost. You'll find the entrance at a depth of roughly 65 feet; just be sure to keep your fins from stirring up silt, lest you add your remains to the heap.