For the intrepid aquatic explorers out there, here are 11 of the world's finest underwater caves.
This coveted cave on Mexico's Yucatan Peninsula is so spectacular it was the object of interest for the film Journey Into Amazing Caves and an episode of Planet Earth. Although it's a snorkeler's haven, it is not for the faint hearted - it is largely inhabited by bats.
Although Florida has been blessed with a bounty of glimmering gems lying underneath the surface, Ginnie Springs is particularly glorious. It beckons scuba divers from far and wide, thanks to its crystal-clear waters and eerie underground passages.
The Grotto at Bruce Peninsula National Park
Waves have carved out the dramatic setting in Ontario's Georgian Bay. The Grotto is the largest and most beguiling of the Niagara Escarpment, boasting striking turquoise-cyan waters.
The Great Blue Hole
The clue is in the name with this natural marvel found near Belize City, in Central America. It is the nexus of the Lighthouse Reef System, and is resemblant of a big, sapphire pupil studding the sea. Professional divers only - the cave waters are known to be surrounded by sharks.
Maldives Underwater Cave
This underwater cave is found near the Dusit Thani in the Maldives. Offering an epic array of flora and fauna, a new type of glowing fish was discovered here - we like to think this diamond in the Indian Ocean is a microcosm of Finding Nemo.