PERTH -?A robotic submarine looking for the lost Malaysian jet began its second mission Tuesday after cutting short its first because the ocean waters where it was sent were too deep, officials said.

Monday's planned 16-hour search lasted just six and none of the data collected by the US Navy's Bluefin 21 submarine offered clues to the whereabouts of the plane.

The unmanned sub is programmed to hover 30 meters (100 feet) above the seabed, but it started searching atop a patch that was deeper than the sub's maximum operating depth of 4,500 meters (15,000 feet), the search coordination center and the US Navy said.

A built-in safety feature returned the Bluefin to the surface and it was not damaged, they said.

The data collected by the sub was later analyzed and no sign of the missing plane was found, the US Navy said.

Crews shifted the search zone away from the deepest water before sending the Bluefin back for Tuesday's mission, the US Navy said.

Search authorities had known the primary search area for Flight 370 was near the limit of the Bluefin's dive capabilities. Deeper-diving submersibles have been evaluated, but none is yet available to help.