In 1977 the first deep-sea hydrothermal vents were discovered at the Galapagos Rift using the submersible Alvin. Where the deep sea is often characterised by low densities of life, these sites hosted spectacular accumulations of species never previously seen, including giant tube worms, clams, mussels, crabs and eelpout fish. These spectacular communities were nestled in rocky crevices formed of basalt and surrounded by shimmering warm water.

It took several years of repeat visits and diligent laboratory work to recognise that these ecosystems were unlike any others seen on Earth. Whereas most life relies on sunlight providing energy for photosynthesis – in the ocean by microscopic phytoplankton and near the coasts, seaweed – at vents, energy for carbon fixation came from the oxidation of chemicals, especially hydrogen sulphide, contained in the vent fluids. In short, the entire food web was based on chemical energy used by microorganisms living free in the environment or in symbiotic association with the animals around the vents. The giant tube worms, for example, had no mouth or gut, instead a finely divided feathery plume which takes up hydrogen sulphide, oxygen and other nutrients and transports them to an internal organ packed full of sulphur bacteria, providing the worm with energy and building materials to grow.

Further exploration identified high-temperature hydrothermal vents billowing clouds of black particles with temperatures exceeding 3,50oC, as well as lower temperature white smokers. Scientists realised that life was possible beyond Earth, in the absence of sunlight and across a wider range of temperatures than previously thought. Overnight, the science of astrobiology (previously called exobiology) was given a huge boost. Deep-sea hydrothermal vents come in many flavours and the discovery of the Lost City vent site on the mid-Atlantic Ridge venting lower temperature alkali fluids from chimneys of talc up to 60m tall produced particular excitement. At this site unusual chemical conditions are thought to resemble those of the Archean Earth when life originated. If such low-temperature vents were the origin of life on Earth, then maybe there has been a second genesis of life elsewhere in the Solar System. Space missions have now detected evidence of submarine hydrothermalism on Enceladus, a moon of Saturn and moons of Jupiter such as Europa and Ganymede also host ice covered oceans.

The discovery of hydrothermal vent ecosystems in the deep sea has changed our view of the possibilities of life elsewhere in the universe. Such a striking discovery may have seemed a one-off. However, the recent announcement of the discovery of dark oxygen production in the deep ocean may be equally momentous. 

Deep-sea mining is a highly controversial subject with industry backers arguing that minerals from the ocean are critical to the renewable energy revolution whilst many environmentalists argue that it could lead to irreversible damage to the biodiversity of deep-sea ecosystems. One of the main deposits of deep-sea minerals lies in the Clarion Clipperton Fracture Zone (CCFZ), located between Hawaii and Mexico in the tropical eastern Pacific covering a vast area of 4.5 million km2, just under half the size of the entire USA. Here the seafloor is deep, between 4,000 and 5,000 metres and includes areas of polymetallic nodules, concretions containing metals such as manganese, copper, nickel, iron and cobalt, which vary in size from small pebbles to roughly the dimensions of a potato.