The true extent to which the ocean bed is dotted with volcanoes has been revealed by researchers who have counted 201,055 underwater cones. This is over 10 times more than have been found before.

The team estimates that in total there could be about 3 million submarine volcanoes, 39,000 of which rise more than 1000 metres over the sea bed.

"The distribution of underwater volcanoes tells us something about what is happening in the centre of the Earth," says John Hillier of the University of Cambridge in the UK. That is because they give information about the flows of hot rock in the mantle beneath. "But the problem is that we cannot see through the water to count them," he says.

Satellites can detect volcanoes that are more than 1500 m high because the mass of the submerged mountains causes gravity to pull the water in around them. This creates domes on the ocean's surface that can be several metres high and can be detected from space.

Data overload

But there is a multitude of small volcanoes that have gone undetected. The only way of identifying them is to manually find their outline on sonar measurements taken from ships.

Since the late 1960s, research vessels have been criss-crossing the oceans using sonar instruments to measure the depth of the ocean floor. They have generated 40 million kilometres of linear profiles showing the topography of the ocean bed between 60° North - the latitude of southern Alaska - and 60° South - corresponding to the tip of Patagonia.

But until now, no one had been able to sift through them all. So, Hillier and a colleague designed a computer programme that was able to analyse the huge amount of data and identify volcano-like shapes in the sonar lines.

The programme found 201,055 volcanoes over 100m tall. Previously, satellite data had identified 14,164 volcanoes over 1500 m high.

Hillier then extrapolated the data to estimate how many volcanoes exist beyond the areas the research vessels sounded out. He estimates there are about 39,000 volcanoes that are higher than 1000 m, leaving nearly 25,000 yet to be directly discovered.

Surprising distribution

Hiller says he was surprised to find that the density of small volcanoes dropped in the area around Iceland, as Iceland is known to be a hotspot for volcanic activity.

Another surprise was that he found fewer volcanoes on the seabed around Hawaii, another volcanic hotspot. He says his findings may mean that researchers need to re-assess their understanding of how submarine volcanoes are formed.

In 2006, a team of researchers from Japan discovered a new type of volcano which also defied conventional theories of volcanism. The "petit-spot" volcanoes, aged between one to eight million years old, did not sit at tectonic plate boundaries or over volcanic hotspots (see New type of volcano fires imaginations).