Illustration: Laura H. Azran

Though he didn’t yet know what the robot would look like, Stone knew exactly where he would test it out: in a series of deep sinkholes, or cenotes, at Rancho La Azufrosa, about 400 kilometers northeast of Mexico City. Scuba divers and geologists have long been fascinated with the site, but detailed studies were lacking.

The deepest cenote, called El Zacatón, held a particular significance. In 1994, Sheck Exley, one of the world’s premier cave divers and a good friend of Stone’s, died while attempting to reach 1000 feet (305 meters). When they pulled his body out of the water, wrapped in his descent line, his dive computer read 268 meters. No human since has succeeded in plumbing Zacatón’s depths.

Limestone cliffs reach up 25 meters above Zacatón’s smooth surface. Flurries of butterflies flit among the wildflowers at the cenote’s lip. Buzzards circle overhead, and the lime-green parrots that live in the cliffs screech at the intruders in their midst. In the afternoon sun, the warm water turns milky, the result of microbes that metabolize the sulfides in the water. After dusk, they’ll release the sulfides again, and powerful fumes will rise up into the night sky.

If Zacatón weren’t pulling double duty as a test bed for advanced robotics, it would be studded with tiny islands of tall grass, called zacate, that float freely on the water. For the robot’s safety, though, the zacates have been corralled at one end behind a yellow rope. A canopied dock festooned like a NASCAR racer with all the logos of the project’s participants sits nearby; during missions, the programmers and robot wranglers use the dock for monitoring the vehicle’s progress. A tall white construction crane perches on one cliff. When the time comes—if the time comes—to fire up the robot, the crane will gently lower the vehicle to the water’s surface.

During earlier runs in January and March, the robot had mapped La Pilita, a smaller, urn-shaped cenote just down the dirt road from Zacatón. The team had returned with high hopes of pushing DEPTHX to its limits, fully exploring Zacatón and perhaps some of the other nearby cenotes.

On its initial run at Zacatón just days earlier, before the snafu over the permit, DEPTHX had descended all the way to the bottom, registering a tentative depth of 318 meters. To put that in perspective: if you submerged New York City’s Chrysler Building in the sinkhole, just the last meter of its elegant spire would stick out of the water.

But the sonar map that DEPTHX drew was tantalizingly incomplete. There appeared to be a passage at the deepest point leading off to one side. Stone has been exploring deep caves in Mexico for decades, and he’s seen passages like this before. “If this is what I think it is,” he says, “it could go on for hundreds or thousands of meters.” The robot would need more time in Zacatón to tell them if the passage was real.

And more time, too, to complete DEPTHX’s scientific mission—namely, exploring the biology of the sinkhole. Zacatón’s walls are covered with a thick slimy coating of micro-organisms, while other microbes float freely in its sulfurous water. The plan is to have the robot gather solid and liquid samples at various depths. Later, DNA analyses and studies of the water’s chemistry would tell scientists much about life in the cenotes. This exercise would also be a vital step for exploring Europa, whose ocean may harbor life.