1. Cheddar Gorge is Britain's biggest canyon and is found within the Cheddar Caves, where the UK's oldest complete human skeleton was found in 1903. Known as the Cheddar Man, the remains were estimated to be 9,000 years old.

2. Hamilton Pool Preserve, in Austin, Texas, was created quite naturally when the dome of an underground cave collapsed revealing the 23 mile-wide pool. It is now frequented by day-trippers and naturalists. That's naturalists not naturists, although no doubt someone has tried to go skinny dipping at one point!

3. Hamilton Pool from another perspective. When there's been heavy rainfall, 45ft waterfalls cascade from the rim of the cavern. It must be pretty spectacular when you're bathing.

4. Stalagtites adorn the roof of Luray Caverns, Virginia, the still waters throwing a perfect reflection.

5. Legend has it that early cavemen inhabited Wookey Caves in Somerset, England.

6. This underground lake in Messalini Caves, near Kefalonia, was found when the roof of the cave collapsed after an earthquake in 1953.

7. Lechuguilla Cave, in Carlsbad Caverns National Park, New Mexico is the fifth longest cave discovered yet at 120 miles (193 km) long and measures 489 metres (1,604 ft) deep, making it the deepest in continental United States.

8. This underground lake near Macan Ché on the Yucatán Peninsula is one of many that are considered to be gifts from the gods by the Mayans, and therefore sacred.

9. The limestone flow feeding into this underground lake in Mexico resembles a waterfall turned to stone.

10. this underwater lake was made by this little waterfall in Banff, Canadamake

11. Reed Flute Cave in Guilin, China was discovered during the Tang Dynasty almost 1,300 years ago.