A California company is working on an underwater refueling station that can top off the fuel cells of undersea surveillance drones, allowing the vehicles to venture farther and work longer. Teledyne, based in Thousand Oaks, California, showed off its undersea power station alongside Gavia, the company’s popular underwater surveillance drone, at the Sea-Air-Space Exposition in Maryland in April. The underwater fuel-cell station stores 200 kilowatts of power and works down to a depth of nearly two miles. An undersea drone could hook up to the station and charge its own fuel cells. A Gavia can operate for up to five hours on one 1.2-kilowatt charge. Teledyne is proposing the seven-inch-diameter Gavia for the Navy’s oceanographic fleet, which maps the sea floor. But the drone, which travels at speeds up to four miles per hour down to a maximum depth of around 10,000 feet, can also help with search-and-rescue and mine hunting missions.