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The discovery was unveiled in a study conducted by the Italian Space Agency. The evidence comes from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument (MARSIS) which is on the European Space Agency's Mars Express spacecraft.

The instrument uses pulses of radar to study the interior structure of the planet and has been orbiting Mars on the Mars Express European spacecraft since 2003.

The scientists explained that radar signals bounce back to the Mars Express in various different ways, depending on what material they have found — and according to the Italian team, the new signals picked up over Mars’ south pole could be explained only by a large underground mass of water.

“We discovered water on Mars,” said Roberto Orosei, a scientist at Italy’s National Institute for Astrophysics and the co-investigator on MARSIS, in a video about the discovery. Orosei said MARSIS “was able to detect echoes from beneath the southern polar cap of Mars that were stronger than surface echoes”.

“This condition on Earth happens only when you observe subglacial water like in Antarctica over places like Lake Vostok,” he added.

The team also suspects that while they have only found evidence for water at one location, it probably isn’t the only subglacial lake on Mars. The ‘lake’ is about 12.5 miles wide, at least 3 feet deep and is located under about a mile of ice.