Deep-sea mountains

On land, you’d struggle to find a mountain that hasn’t already been climbed. In contrast, in the deep sea there are thousands of unexplored peaks. Seamounts are submerged volcanoes, active or dormant, with foothills planted in the abyss and summits soaring up thousands of metres without breaking the sea surface.

These hidden mountains are some of the least known, but most abundant geological features on the planet. They form a fragmented habitat that covers an area rivalling the world’s tropical rainforests. As scientists learn more about seamounts, it’s becoming clear that these dramatic montane seascapes are rich oases of life that play a crucial role across the entire global ocean.

Currently, there’s no definitive count of the world’s seamounts, because locating and identifying them is not easy. Estimates suggest there are between 30,000 and more than 100,000 seamounts with peaks over 1,500m high. One of the biggest is Davidson Seamount off the California coast – 42km long, 8km wide and 2,280m tall. Taller still are seamounts that rise almost 5,000m from base to peak. Add in smaller peaks, 100m and higher, and the estimated global tally reaches into the millions. Whatever the number of seamounts, scientists have studied only a few hundred. Dr Lucy Woodall is a senior research fellow at Oxford University and principal scientist at the research foundation Nekton, who has studied seamounts in the Atlantic, Indian and Southern Oceans. “It’s something I think about before every dive, that I’m probably the first human to see this bit of our planet, just because they’re so remote and so unexplored,” she says.

When exploring seamounts, scientists often encounter otherworldly forests of sponges and corals, including colourful, shrub-like colonies of gold and black corals that live for hundreds or even thousands of years. Deep-dwelling coral species already outnumber their distant relatives in the tropical shallows and new species are constantly being found. A recent expedition to the Galapagos Marine Reserve uncovered dozens of new species of corals and sponges growing on three previously unexplored seamounts.

Amid the expanses of mud-covered deep-sea floor, the rocky flanks of seamounts provide a footing for larvae of corals and sponges to settle on and grow. The corals and sponges then offer a habitat for other animals: starfish, anemones, snails, brittlestars, shrimp, squat lobsters and octopuses. Sharks lay their egg cases among the coral branches like Christmas tree decorations.

Long-lived corals also keep a record of how the ocean has changed. By extracting traces of certain chemicals and measuring isotopes, scientists can estimate the temperature, pH and nutrients of seawater when different parts of the coral colony grew, some more than 4,000 years ago.

Science Focus