The Bluefin-21 and its "side scan" sonar has become the focal point of the search 2,000 km (1,200 miles) west of the Australian city of Perth, where authorities believe Malaysia Airlines Flight MH370 hit the ocean after disappearing from radars on March 8 with 239 people on board.
Now in its seventh week, the search has centred on a city-sized area where a series of "pings" led authorities to believe the plane's black box may be located. But after almost two weeks without a signal, and long past the black box battery's 30-day life expectancy, authorities have turned to the Bluefin-21.
After the $4 million Bluefin-21's searches were frustrated by an automatic safety mechanism which returns it to the surface when it exceeds a depth of 4.5 km (14,763 feet), authorities have adjusted the mechanism and have sent it as deep as 4,695 metres (15,403 feet), a record.
But hopes that the Bluefin might soon guide searchers to wreckage are dwindling with no sign of the plane after six deployments spanning 133 square kilometres (83 square miles). Footage from the Bluefin's sixth mission was still being analysed, the Joint Agency Coordination Centre said on Saturday.
Malaysian acting Transport Minister Datuk Seri Hishammuddin Hussein said in a Twitter post that the government's Deployment of Assets Committee was considering using more autonomous underwater vehicles. He did not elaborate.
On Monday, the search coordinator, retired Air Chief Marshal Angus Houston, said the air and surface search for debris would likely end by midweek as the operation shifted its focus to the ocean floor.
But the air and surface searches have continued daily, and on Saturday the Joint Agency Coordination Centre said up to 11 military aircraft and 12 ships would help with the day's search covering about 50,200 square kilometres (31,000 square miles) across three areas.