Actually, gone. Imploded. BBC has more on the tragic end of the deep-sea submersible Nereus:
The robotic vehicle Nereus went missing while exploring one of the ocean’s deepest spots: the Kermadec Trench, which lies north east of New Zealand.
Surface debris was found, suggesting the vessel suffered a catastrophic implosion as a result of the immense pressures where it was operating some 10km (6.2 miles) down.
…
“Nereus helped us explore places we’ve never seen before and ask questions we never thought to ask,” said Timothy Shank, from the Woods Hole Oceanographic Institution (WHOI), which managed the sub’s activities.
“It was a one-of-a-kind vehicle that even during its brief life brought us amazing insights into the unexplored deep ocean, addressing some of the most fundamental scientific problems of our time about life on Earth.”