The source of El Nacimiento (â€The Springâ€) lies just 100 meters west of the main field lab at Zacaton. The horizontal black slot at center is the entrance to the Pasaje de la Tortuga Muerta (â€Dead Turtle Passageâ€), which leads upstream and underground to the main Zacaton cenote. It was first explored in 1989 by Jim Bowden and Gary Walton and leads 220 meters underground, re-emerging at the southwest corner of Zacaton.