The Mayan Riviera on Mexico's Yucatan Peninsula may look like your typical beachside holiday destination but for spelunkers, cave divers and anyone with a spirit of adventure, its beauty is more than skin deep.
Behind the beachfront resorts, the tropical jungle is home to the longest underground river on earth - all 95 miles (153 kilometers) of it. In places the earth has collapsed into the waters below, forming cenotes, deep wells of gin-clear water that provide an entry into the surreal underworld.
Which river will suit you?
For claustrophobes, Xel-Há is the best place to experience the action: entry into the river is as easy as lying back in an inner tube where the underground waters emerge and following the current downstream under the reassuring heat of the sun above.
The more intrepid adventurer with specialist cave diving certification might prefer The Pit: to enter, divers brave an 18 foot (six meter) jump into to the cenote before donning scuba gear and vanishing into the darkness. Most visitors to the Yucatan Peninsular favor something in between.
Top adventure experiences
For an underground experience high on thrills but (relatively) low on terror, these adventure experiences stand out: Hidden Worlds, Rio Secreto, Alltournative and Dos Ojos (home of The Pit and a range of other less terrifying dives).
To enter Hidden Worlds, a rollicking drive through the jungle to the starting point in a stripped down troop carrier is part of the fun. This non-profit adventure park features cenotes as a splashdown for a series of ziplines, including one rollicking 'rollercoaster zipline' called Avatar. (Safety harness and whoops of joy are mandatory.)
Rio Secreto's underground river, meanwhile, is shallower and largely navigated on foot through waist-deep water. Visitors gear up with wetsuits, neoprene boots, helmets and headlamps for the two-long guided tour. One of the best aspects of the cenote adventure parks in this area is their rugged authenticity - there are no brightly-lit boardwalk tours with cheesy commentary to be found. Instead, knowledgeable guides lead visitors through caves dripping with stalagmites and swim through deep pools to experience a taste of true spelunking. The cave system abounds with 12ft-wide chandeliers of stalactites and rarer helictite formations which encrust the icicle-like structures in a thick coat of what looks like snow.
While there is some swimming involved, Rio Secreto's focus is most definitely overhead. For an entirely different perspective it pays to slip on a mask and snorkel. In the cave system explored by adventure outfitter Alltournative guests swim in Yax Mul and Nohoch, the very cave explored in the BBC documentary series Planet Earth.
The experience is magical and adventurous in equal measure. In places the overhead clearance is a matter of inches while below, the cave is as vast and ornate as a ballroom. In Yax Mul beams of sunlight filter through tiny cracks in the surface softly illuminating the water with an eerie blue light. (The fact that natural light penetrates through cracks and holes qualifies Yax Mul as a cavern rather than a cave.)
Back at the surface the thick jungle gives no hint as to what lies beneath. It's remarkable that one of Mexico's most incredible sights – the underworld of the Maya - is hidden from view. It's a wonderland well worth donning a wetsuit for.