Yucatan geology 101 - the Yucatan peninsula (south-east part of Mexico) sits ontop of a massive slab on limestone. As such, there are no rivers on the surface - rainwater almost immediately drains through the limestone to an underground river system beneath. As the water erodes to limestone above, the rock collapses to form big caves (sometimes open to air, sometimes not) called cenotes, which are dotted throughout the jungle and form one of the biggest cave networks in the world. Infinitely preferring freshwater over seawater, this sounded like just my bag. So I came here (here being Tulum) from Cozumel on Monday. Once I had checked into my hostel, I went out for a bit of a gander round the town, but given that involved a 15 minute bike ride along a major highway,
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