Teams searching for a missing Malaysian airliner are planning for the first time to send an unmanned submarine into the depths of the Indian Ocean to look for wreckage, an Australian official leading the search said Monday.

The deployment of the submersible drone opens a new phase in the five-week search, one focusing on a pitch-black, silt-covered patch of the ocean floor. The drone moves at walking speed, and searching with it will be painstaking. But it allows investigators to follow up on their best lead to date: deep-sea acoustic signals that have come from the Boeing 777's black box.

The submersible drone, known as the Bluefin-21, will use sonar to provide search crews with a three-dimensional map of the Indian Ocean floor, an area so unexplored that it is practically "new to man," said Angus Houston, the Australian official in charge of the search.

Thirty-eight days into the search for Malaysia Airlines Flight MH370, which vanished with 239 passengers and crew on board, there have been no confirmed sightings of debris. The best hint of the plane's location comes from four acoustic signals - potential black-box transmissions - that were detected by an Australian naval vessel equipped with specialized listening equipment. The last transmission, though, came six days ago, and the aircraft black-box batteries are already well past their 30-day shelf life. The batteries by now could have died, Houston said.

"I would caution against raising hopes that the deployment of the autonomous underwater vehicle will result in the detection of aircraft wreckage," Houston said. "It may not. However, this is the best lead we have.

"We've got to find wreckage before we can finally say we've solved this mystery."

The Bluefin-21 is already on board the Australian vessel, the Ocean Shield, operating roughly 1,050 miles northwest of Perth. For more than a week, the Ocean Shield has been dragging a towed pinger locator through the water to listen for possible pings from Flight MH370's black box.

Separately, Houston said Monday, the Ocean Shield also came across an oil slick about 31/2 miles downwind from the area where it picked up the signals. Though the oil slick could be unrelated to the plane, about half a gallon has been sampled for analysis - a process that will take several days, Houston said.

The U.S. Navy said the submersible will need anywhere from six weeks to two months to scan the underwater search area.