The morning begins before the bus sighs out of San Ignacio: humidity clings to the windows, the driver navigates a ribbon of road that slips between citrus groves and shade trees, and the jungle seems to lean forward as if eavesdropping. After roughly an hour you enter Tapir Mountain Reserve, where a short 30-minute jungle hike delivers you to the mouth of Actun Tunichil Muknal — the ‘‘ATM’’ cave — a river-carved vault of rock and ritual. Inside, water dictates the pace; you wade, scramble over calcified ledges and stop where cathedral-like chambers open above.