Veteran cave explorer Bill Stone has announced that in 2017, he'll lead an expedition into the Chevé Cave system, a sprawling underground complex in the Oaxaca region of Mexico, which water flow suggests may descend nearly 1.6 miles into the Earth. If that turns out to be true, Chevé would earn the title of the world's deepest cave, taking away the distinction currently held by Krubera Cave in the Western Caucasus mountains in Georgia, which is about 1.36 miles deep.
But scientists say that even if Chevé sets a new depth record, it may not last. Geological data leads them to believe there are numerous undiscovered caves across the globe, and that some of of those subterranean passageways may reach vastly deeper into the planet. Given the limits of the technology used for detection, though, finding those caves may be a challenge, and exploring them even more so.
There are plenty of places to look for them. Karst terrain -- rugged landscapes with high elevations and and underlay of limestone, into which water can seep to carve out caves -- covers 20 to 25 percent of the Earth's land surface. "There are probably tens of thousands of undiscovered caves that might exist out there," explains George Veni, executive director of the National Cave and Karst Research Institute.
At least in theory, some of those caves may reach far deeper than any humans have explored, according to Lewis Land, a hydrologist for the institute. "The only limit is how far down groundwater can circulate though limestone, before the pressure becomes too great," Land said. "And it seems that it can do that at far greater depths than we once thought."
The Kola Superdeep Borehole, a Soviet-era experiment to drill deep into the Earth that lasted from 1970 until 1994, reportedly found water circulating at a depth of 4.3 miles. Land said that in the 1980s, an oil exploration project in Oklahoma broke through into a cave that was at least 2.2 miles beneath the Earth's surface. That sort of subterranean chamber, which has no entrance anywhere near surface level, would be difficult if not impossible for humans to explore.
Other super-deep caves might be accessible, provided researchers can locate them. Right now, the means to do so are limited. The sensing technology that's most readily available to cave explorers is electrical resistivity, in which an instrument measures how electricity moves through the ground, and looks for subtle fluctuations that indicate a cave beneath the surface.
But that method is only useful about to a depth of about 800 feet, Land said. Seismic reflection, a technology used by energy companies to prospect for oil and gas deposits, can reach deeper, but it doesn't have the resolution to spot a passageway that may only be a few feet wide.
And while space scientists can study distant worlds with satellites equipped with robotic instruments, it's harder to do that when you go underground.
"Remote sensing technologies are very limited with caves,"Joel Despain, chairman of the International Exploration Committee of the National Speleological Society of the U.S., explained in an email. "In some circumstances thermal imaging can reveal cave entrances with airflow that is cold or warm compared to surface temperatures."
That's why cave explorers rely on a more low-tech but still reliable method. "Cave researchers dump non-toxic dye into cave streams to see where the water emerges at springs far downstream and at a much lower elevation," Despain said. "This is what ultimately determines the potential depth of Cheve and most caves -- the height of where cave-making-water enters the cave system and the depth of where it exits and emerges as a spring on the surface."
But even when a dye trace indicates what might be a record-deep cave, that's just the start of the challenge of exploring it.
"Just because the water can flow to that lowest spring there is no guarantee that any human can follow," Despain explained. "They will need to find their way through faults that turn huge spacious cave passages into giant mazes of tiny holes between giant unstable blocks of rock, and they will dive into water-filled passages using very advanced equipment where there is no surface to swim up to if there is an equipment problem. They will rig thousands of meters of ropes to reach the great depths of the cave and will manually haul and carry all of this equipment to the bottom."
"It is a huge, complex and very challenging and risky endeavor," Despain said.
The only real way to determine a cave's depth is for humans to get to the bottom, and there's no guarantee that they will be able to probe deep enough into Chevé to set the record. Alexander Klimchouk, a researcher at Ukraine's Institute of Geological Sciences and co-leader of the group that explored Krubera in 2004, wrote in an email that cave hasn't been fully explored, either, and that it might turn out to be at least as deep as Chevé's projected size.
"However, I respect the dedication of Bill Stone and the team (this is what drives all discoveries), and I wish them good luck," Klimchouk said.
Regardless of which cave turns out to be the deepest, science will win because these deep spaces in the Earth contain a potentially vast amount of information for researchers. Veni noted that caves are filled with living organisms, including insects and microbes that may help scientists to discover new antibiotics and other medicines.
They also have preserved evidence of past climate cycles, which scientists can use to fine-tune their models for future trends. Additionally, NASA is interested in studying caves on Earth so that it can develop technology to explore them on Mars or other worlds.