A seagoing glider that uses heat energy from the ocean to propel itself is the first "green" robot to explore the undersea environment, United States researchers said.
They said the glider had crisscrossed the 4000m deep Virgin Islands Basin between St Thomas and St Croix more than 20 times since it was launched in December.
And it could keep going on its own for another six months, the team at the Woods Hole Oceanographic Institution and Webb Research Corporation in Falmouth, Massachusetts, predicted.
"Gliders can be put to work on tasks that humans wouldn't want to do or cannot do because of time and cost concerns," Dave Fratantoni of Woods Hole said.
"They can work around the clock in all weather conditions."
Such robots can carry sensors to measure temperature, salinity and biological productivity. They usually surface from time to time to fix their positions using the Global Positioning System and to communicate via Iridium satellite to a laboratory.
Most gliders rely on battery-powered motors and mechanical pumps, the researchers said. This one draws its energy from the differences in temperature between warm surface waters and the colder, deeper layers of the ocean.
"We are tapping a virtually unlimited energy source for propulsion," Fratantoni said.
Data collected by the glider would help researchers understand how eddies in the region affect ocean circulation and move around the larvae of fish as well as pollutants, he said.