Opalescent pools full of carbon dioxide have been found at the site of the second biggest volcanic eruption recorded in human history.
The eruption in the Aegean Sea off the coast of Santorini wiped out the Minoan civilisation living along the coast in 1600 BC.
The newly discovered pools were found forming at a depth of 250m. They is a series of interconnected white pools that have high concentrations of CO2 and scientists say they could shed light on future volcanic eruptions and answer questions about deep sea carbon storage.
An international team of scientists used sophisticated underwater exploration vehicles to find the pools, which they have named the Kallisti Limnes, from ancient Greek for "most beautiful lakes".
Rich Camilli, lead author the study published in the journal Scientific Reports, said: "The volcanic eruption at Santorini in 1600 BC wiped out the Minoan civilization living along the Aegean Sea. Now these never-before-seen pools in the volcano's crater may help our civilization answer important questions about how carbon dioxide behaves in the ocean."
The investigation into the site was prompted by concerns about the volcano in 2011. During a preliminary search, an autonomous underwater vehicle identified a number of subsea layers with unusual chemical properties.
Subsequent manned missions allowed scientists to analyse the chemical signature of the water column along the caldera – which is when they came across the opalescent pools.
Santorini is the most active part of the Hellenic Volcanic Arc and during subduction of the African tectonic plate, CO2 can be released by magma degassing or from sedimentary materials altering as a result of the huge pressure and extreme temperatures.
"We've seen pools within the ocean before, but they've always been brine pools where dissolved salt released from geologic formations below the seafloor creates the extra density and separates the brine pool from the surrounding seawater," Camilli said.
"In this case, the pools' increased density isn't driven by salt – we believe it may be the CO2 itself that makes the water denser and causes it to pool."
Previously it had been thought that when CO2 is released into the ocean, it disperses into the surrounding water. Camilli said the two fluids they found remain separate, with the denser CO2 water sinking and forming the pool.
This, they say, has implications for the build up of CO2 in areas with little circulation – including the Kolumbo underwater volcano nearby that is completely enclosed. "Our finding suggests the CO2 may collect in the deepest regions of the crater. It would be interesting to see," Camilli said.
They also say monitoring the temperature and chemical signatures of the pools could be used along with other monitoring techniques to watch for increased or decreased volcanism – the Kallisti Limnes were 5C above those seen in the surrounding waters.
Study co-author Javier Escartin said: "This heat is likely the result of hydrothermal fluid circulation within the crust and above a deeper heat source, such as a magma chamber. Temperature records of hydrothermal fluids can show variations in heat sources at depth such as melt influx to the magma chamber. The pool fluids also respond to variations in pressure, such as tides, and this informs us of the permeability structure of the sub-seafloor."