Kentucky’s Mammoth Cave National Park is the world’s longest known cave system. It’s at least 400 miles (644km) long, but there’s so much more that hasn’t even been explored.
Postojna Caves is Slovenia’s largest cave system, which spans over 12 miles (19km), with a quarter of that open to the public.
Then make like R2D2 and head to Hotel Marhala in Matmata, southern Tunisia. Some people in the Berber-speaking town still live in these traditional 'troglodyte' caves.
While there are plenty of reasons to visit Luray – huge caverns, an incredible lake, ceilings that are 10 storeys high – the main reason is to hear the world’s largest musical instrument.
It’s not exactly a top tourist attraction, but Turkmenistan’s Door to Hell is pretty fascinating. Officially called the Darvaza crater.
No one is really sure how Margate’s shell grotto came to be. It was discovered in 1835 by James Lovelock and his son, Joshua.
Ice caves appear and disappear each year in Alaska's retreating Mendenhall Glacier and are notoriously difficult to reach.
The view at Grotta Palazzese restaurant is nothing short of perfection. It’s built into a natural cave overlooking the Adriatic and the sunsets are as gorgeous as you’d imagine.
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