ABE (Autonomous Benthic Explorer), which pioneered so many discoveries from deep sea was lost in the depths of pacific. (http://blogs.discovermagazine.com/80beats/2010/03/11/pioneering-deep-sea-robot-is-lost-to-a-watery-grave/)
This robot was instrumental in mapping the mid-ocean ridge and studying the strange life that flourish in the volcanic vents.
Deep sea exploration (well, easy chair exploration in my case) is such a fascinating area. The secrets deep ocean still keeps are as big as any other secrets in the universe.
It is a shame that the resources we spend on exploring the seas is meager compared to what is spend on space exploration. The technical challenges it faces are much more than space exploration as well.
Robots like ABE and its newer cousins are valuable assets to scientific exploration and in understanding the largest areas of our own earth. Wired magazine called it one of the 50 best robots ever.
The robot is assumed to have succumbed to its death due to an implosion of its glass domes followed by a cascade failure of other domes.