Nereus was a stalwart of deep sea exploration, and a prized tool of U.S. science.
"Nereus helped us explore places we've never seen before and ask questions we never thought to ask," Timothy Shank, a scientists at the Woods Hole Oceanographic Institution (WHOI), told BBC News. WHOI managed Nereus's research and diving activities.
Nereus lost contact with controllers while cruising at a depth of 6.2 miles beneath the ocean's surface. Debris from the sub, later found floating in the area, led scientists to believe Nereus suffered an implosion.
At the time of its demise, Nereus was exploring the second-deepest ocean trench on earth. The underwater pressure was a crushing 16,000 pounds per square inch, or psi.
"Extreme exploration of this kind is never without risk, and the unfortunate loss of Nereus only underscores the difficulty of working at such immense depths and pressures," explained Larry Madin, the head of research at WHOI. "Fortunately, there was no human injury as a consequence of this loss."
There was financial loss, however -- $8 million. Launched in 2008, Nereus was constructed with funding from the National Science Foundation, the Office of Naval Research, the National Oceanic and Atmospheric Administration, as well as several non-profit organizations.