The mystery surrounding missing Malaysia Airlines Flight MH370 has deepened as the autonomous underwater vehicle Bluefin-21 wraps up its search without finding any debris and the search area has been discounted as the final resting place of the ill-fated aircraft.

The Joint Agency of Coordination Center announced on Thursday that Bluefin-21 had completed its mission after covering over 850 square kilometers of ocean floor in an area some 1,600 km off the coast of Western Australia.

The area was defined based on the four acoustic signals detected by the towed pinger locator deployed by the Australian Defense Vessel Ocean Shield. The signals, or pings, were believed to be from a man-made device and have all the characteristics of the transmission from an emergency locator beacon.