Oil companies, navies and oceanographers are always trying to sharpen their picture of the ocean floor. Until now, they've relied largely on surveys performed by remotely operated vehicles, tethered by cables to ships. But the tethers restrict how deep and fast the unmanned vehicles can go. Now, thanks to advances in artificial intelligence, a few companies are building completely cable-free autonomous underwater vehicles that can gather data on explosive mines or new offshore oil-drilling sites faster and more cheaply than remotely operated vehicles.

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