On May 9, one of the world’s most advanced submersibles Nereus, blew apart 10 km deep underwater. The submersible, operated by the US Woods Hole Oceanographic Institution (WHOI) was on a deepsea expedition and the loss of the unmanned exploratory under sea vehicle has left ongoing and future projects high and dry. It is believed that It is likely that one of the ceramic vessels used to encase components on Nereus collapsed under the water pressure, leading to an implosive chain reaction. A report on the mishap appeared in a recent issue of Nature. The loss gains significance because Nereus was designed to explore as part of a US National Science Foundation (NSF) programme, the hadal zone. This area of the ocean, in deep-sea trenches between 6,000 and 11,000 metres down, is one of the least explored regions on Earth.

Researchers have long dismissed the idea that the area is a ‘dead zone’. For example, on its first deep dive in May 2009, Nereus discovered a new species of anemone in the deepest part of the ocean, Challenger Deep in the Pacific’s Mariana Trench. But systematic exploration has been lacking, leading in 2011 to the creation of the NSF project, an international collaboration called the Hadal Ecosystem Studies (HADES) programme. It aims to determine the composition and distribution of hadal species, and the role of hadal pressures, food supply and trench topography on community structure.

Tim Shank, a biologist at the WHOI in Massachusetts is the chief scientist on the expedition. During the final dives of Shank’s expedition, the first for HADES, researchers were measuring the metabolic rates of invertebrates such as sea cucumbers. This was done by using Nereus to place the creatures in a respirometer mounted on a separate platform. The work, which began in April, would have provided crucial data on whether trenches act as carbon sinks, and would have enabled researchers to start unpicking the food webs of the deep ocean. A blog posted on The Internet by 0ne of WHOI scientists explains how during a dive to 4,000 metres, Nereus ran a respiration experiment to measure how much oxygen different deep-ocean organisms breathe over time. Five small invertebrates were captured and deposited in containers in a basket. After attaching a lid over the top of the basket, the entire apparatus, known as a respirometer was lifted to various levels from the seafloor and oxygen levels, a proxy for metabolism were measured. The study tries to answer how metabolic rates differ among species as they descend into the trench and calculate a rough carbon budget to provide an answer to the question whether trenches are sinks for carbon.The loss is a blow to oceanographic research because Nereus, until the tragedy, was the only scientific vessel capable of operating at such a depth (11,000 metres down), essential to study Hadal species.Scientists hope to continue with the project by collecting animals, images and sediments from the hadal zone, using baited traps and instrument platforms, called landers, that sink to the bottom. Also being investigated is deploying a towed camera system. However, these are not the same as pointing cameras directly at objects of interest in real-time access as was being done using Nereus.

There is a glimmer of hope for deep-sea exploration with submersibles. Even before the demise of Nereus, the WHOI was in discussions with the Schmidt Ocean Institute in Palo Alto, California, about constructing another ‘full-ocean capable’ vehicle. The collaboration is in a preliminary design phase. Scientists could be using the vehicle by mid-2016. The WHOI is in contact with its insurers over Nereus. A payout could be used to build two of the next-generation vehicles envisaged, instead of one.