U.S. scientists plan to use a self-guided robot submarine to probe the world's deepest sinkhole after "impressive" tests in Mexico in February.

The NASA-funded Deep Phreatic Thermal Explorer (DEPTHX) will be used to explore the Zacatón geothermal sinkhole, or cenote, in the Mexican state of Tamaulipas in May.

No one knows how deep Zacatón goes. Divers have reached a depth of 282 metres — well below what is considered a safe zone — without finding the bottom and sonar does not work within the enclosed space over long distances. Sonar has proven effective only to a depth of about 270 metres.

NASA hopes the mission will lead to technologies that it could potentially use to explore the oceans under the icy surface of Europa, one of Jupiter's moons.

Robot subs typically use a tether or cable to connect to a surface station to draw power, receive commands and send back data. But at the extreme depths being explored, the risk that a cord could become snarled or get caught on a geological protrusion led to a decision to design DEPTHX to be self-contained and operate on its own.

The February test in the 115-metre deep La Pilita sinkhole, also in Tamaulipas, Mexico, proved that the craft can operate successfully, researchers said.

"The fact that it ran untethered in a complicated, unexplored three-dimensional space is very impressive," said mission leader Bill Stone, an engineer and cave diver who heads Stone Aerospace Inc. of Austin, Texas. He noted that the accomplishment is something that has never been previously demonstrated in autonomous underwater vehicles (AUVs).

DEPTHX uses mapping software written by researchers led by David Wettergreen, associate research professor at the Carnegie Mellon University Robotics Institute in Pittsburgh, Penn.

In mapped areas, the sub navigates using depth, speed, direction and inertial guidance, while in unknown territory it uses its array of 56 sonar sensors to navigate by detecting obstacles and locating itself on a software-generated map.

Carnegie Mellon PhD student Nathaniel Fairfield adapted a 2-D program into the 3-D simultaneous localization and mapping (SLAM) software DEPTHX uses to navigate in all three dimensions.

Test in La Pilita showed that the sub could determine its position to within 15 centimetres even after operating independently for hours underwater.