According to Cosmic Variance,one of the Discover blogs, a new robot sub was the third robot to visit the Mariana Trench.
They don’t mention how long it stayed. But it might help Washoe fulfill his dream.
The other robots in the list are pretty cool, too. The Nereus was #12 of 12 robots listed. Not sure how I feel about the swallowable robot that can check your colon.