What is at the bottom of the Mariana trench ?

The Mariana trench is the deepest point in the oceans. Filmmaker James Cameron of Avatar and Terminator fame, in 2012 in his custom-built submarine, the Deepsea Challenger, went the 7 miles down, alone to the bottom of the trench. He witnessed things he could not explain so didn't mention what he saw till recently. Just in time for sea change in belief in "aliens" and UFO's. Two years later after Cameron went down, a research vessel over the trench recorded a strange sound.