Fingal's Cave is in the uninhabited island of Staffa in Scotland: The hexagonal columns are basaltic rock similar to the formations found in The Giant's Causeway in Ireland
Many 'sea caves' are famous among the watersports community - largely because you have to strap on a pair of flippers and an air tank to explore sights such as Belize's Great Blue Hole.
But the relentless action of the waves over millennia has carved out some spectacular caves that you don't even need a snorkel to explore such as Fingal's Cave in Scotland - just a sturdy pair of boots.
The hexagonal basalt columns in Fingal's cave are similar to the formations found in the Giant's Causeway in Ireland
Apostle Islands Ice Caves in Wisconsin's Lake Superior: When the lake freezes over in the winter, so do the caves - visitors venture inside to see frozen lakes and frozen waterfalls
Fingal's Cave, on the uninhabited island of Staffa in Scotland, showcases unique, unspoiled geography - and was reputed to be home to a giant who built a causeway between Scotland and Ireland.
Other caves (below) are reputed to have been home to the Old Man of the Sea - or even sea- God Neptune himself.
A 'littoral sinkhole' - a collapse into an undersea cave - in Santa Rosa California. Such features are created when the roof of a sea cave collapses due to erosion - but can become tourist attractions in their own right
The Belvedere 'upside-down' hole in Sardinia - myth says that it's the home of the Old Man of the Sea. His neighbour is even more illustrious - Neptune, god of the sea, lives in a cave just 100m round the corner
The 'Great Blue Hole' - Belize. The popular scuba-diving destination is 407 feet deep
The 'Sea Lion Cave' in Oregon is among the largest complexes of underwater caves in the world, inhabited by a variety of sea life including sea lions. Entrance, sadly, is via a gift shop