We loaded up for the third site, where the owner waited for us at the locked gate off the road. Santiago waved us through, then closed up and walked to his Jeep. He led us a few miles into the jungle, stopping in a small clearing. Linda got out and started filming as he greeted us. "The Cenote is down this trail," he said. "I'm too old for it, but you kids have fun."

We geared up as Juan gave us the brief. "This Cenote is a pit-type, with sheer walls and almost a hundred feet deep," he said. "There is a layer of hydrogen sulfide at sixty feet that...