He entered an underwater portion, facing a rushing current he compared to a waterfall. He had to plant his feet firmly or the current could have swept him away. And he had to position his head straight into the current, or the water would rip the mask off his face.

At one point, he swam underwater for about an hour, in a 100 meter stretch near a fork in the cave. The murky, muddy water had “zero” visibility, he said. Even when Mr. Ruengrit put his hand in front of his facemask, he couldn't see anything at all.