PERTH, Australia (AP) — A robotic submarine headed back down into the depths of the Indian Ocean on Friday to scour the seafloor for any trace of the missing Malaysian jet one month after the search began off Australia’s west coast, as data from the sub’s previous missions turned up no evidence of the plane.

It was the fifth attempt by the Bluefin-21 unmanned sub to find wreckage or the black boxes from Flight 370 in a distant patch of seabed. The sub, which can create sonar maps of the ocean bottom, has now covered 110 square kilometers (42 square miles) of the silt-covered seabed, but has thus far found nothing, the search coordination center said. The sub’s last mission hit a record depth beyond its recommended diving parameters, which can potentially cause risk to the equipment, the U.S. Seventh Fleet said in a statement. However, it is being closely monitored.