Researchers abruptly lost contact with the $1.1 million robot Friday after it reached the Pacific Ocean floor, nearly 2 miles down.
Scientists believe a glass sphere that helped keep the robot buoyant may have imploded under the water pressure, destroying onboard communications.
It was the robot's 222nd research dive.
The robot was known as ABE. It was made at the Woods Hole Oceanographic Institution on Cape Cod and was launched in 1995.
ABE had greater range and technical abilities than human-occupied submersibles or vehicles connected by cables to surface ships. It was the first autonomous robot to make detailed maps of mid-ocean ridges and locate hydrothermal vents, where hot liquid spews from the ocean floor.