The paddle is soft and slow, the canoe cutting a mirror through water that has flowed the same route for millennia. Trees close overhead until the jungle becomes a vault and the sky reduces to a strip of silver; downstream, walls of limestone rise into cathedral ceilings dotted with calcite formations. This is Barton Creek Cave — the kind of place where the present borrows the atmosphere of the ancient. By afternoon, the sound changes: you trade the hush of the cave for the thunder of Big Rock Falls, where water drops into a deep cenote and sends a cool mist that dares you to stand under the curtain.