We could make an argument that the frontier for physical exploration is underwater caves. In fact, the deepest point accessible to humans likely sits in a water-filled sump at the bottom of a deep cave, accessible only by diving. Underwater caves need to be explored by people because only people can navigate them properly. These caves are 300 feet below the surface and go back miles and miles. Robots get stuck and lost and can’t collect samples because they don’t have the dexterity.

The race to this place is far from over. As teams are pushing further underground, they are faced with the most challenging diving conditions on the planet. Some of these underwater caves can be found inland in Mexico, while others, like those in the Bahamas, are located in parts of the ocean bottom that were once dry during past ice ages.

As dive gear, including mixed-gas rebreathers, underwater scooters, heated drysuits, and decompression computers become more reliable, divers are matching the technology with bolder missions to this inner space. Traveling over 100 meters deep in complete darkness with a thin nylon guideline as the only link to the world that we have evolved to live in, the mental and physical challenges are unmatched.

The booty that we’re after are never-before seen life forms called extremophiles, which can tell us loads about the evolution of microbes that dominated the earth billions of years ago. We may find rare fossils from previous ice ages when the caves were dry, or geologic samples providing insight into the earth’s climatic past—which may tell us what’s coming down the pipes as we pump greenhouse gasses into the atmosphere at unprecedented speed.