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Mount Etna, one of the world鈥檚 most famous volcanoes, may be misunderstood. According to one geologist, the material feeding the cone is mostly water, so Etna is effectively a giant hot spring. But other geologists are unconvinced.

Mount Etna in Italy is almost constantly active. It鈥檚 been estimated that . But what really puzzles at the University of Catania, Italy 鈥� about 30 kilometres from the volcano 鈥� is that Etna also belches out .

The conventional explanation is that this gas bubbles out of magma as it loses pressure on its way up through the volcano鈥檚 vent. But Ferlito says Etna would need to erupt ten times more lava than it does to account for all the gas that burps out.

Alternatively, maybe most of the molten rock in Etna loses its gas and sinks again, without erupting. But Ferlito鈥檚 calculations suggest that sustaining the gas emissions would require a fresh injection of 10,000 kilograms of magma every single second. This would 鈥渋nflate the volcano like a children鈥檚 balloon鈥�, he says.

Water, water everywhere

So instead, Ferlito argues the easiest way to explain Etna鈥檚 excess gas is to ditch the idea that it is fed only by magma. He has calculated that the volcano鈥檚 deep plumbing system could hold lots of water, carbon dioxide and sulphur, collectively making up about 70 per cent of the volume of material feeding the volcano. 鈥淥nly 30 per cent is molten rock,鈥� he says.

Such a system is closer to a hot spring than to a volcano as we understand it.

Ferlito suggests there are water-rich pockets inside the Earth, independent of molten rock, that can feed volcanoes like Etna. In line with this, he points to the growing evidence for unexpectedly large quantities of water deep inside the Earth.

However, while this solution is 鈥渋nventive鈥�, there are simpler alternatives, says at Arizona State University in Tempe.

In 2015 she suggested much of the excess gas originates much deeper in Earth鈥檚 interior, from churning molten rock. It could then percolate up and feed the volcano (). In this scenario, there鈥檚 no need for Etna鈥檚 plumbing system to be injected with thousands of kilograms of fresh magma each second.