Sinkholes, underground swimming and staircases that lead to the centre of the earth, Oman is a fantasy of subterranean sites. The Al Hoota Caves is one of the world’s largest cave complexes. Take the stalagmite staircase down to the underground lake where blind fish flounder in the dark. Next, seek out sublime wild swimming at the Bimmah Sinkhole. Locally named the House of the Devil, the jade blue waters are deliciously cool and clear, and the shimmering desert creates a magical mirage.