Scientists have discovered a reservoir of liquid water on Mars, deep within the planet's rocky crust.

The findings come from a new analysis of data from NASA's Mars Insight lander, which landed on Mars in 2018.

The lander carried a seismometer that recorded four years of vibrations deep inside the Red Planet - Martian quakes.

Analyzing these earthquakes - and the exact way the Earth moves - reveals the "seismic signature" of liquid water.

While there is frozen water at the poles of Mars and evidence of water vapor in the atmosphere, this is the first time liquid water has been discovered on Earth.

The findings were published in Proceedings of the National Academy of Sciences.

InSight's science mission ends in December 2022, after four years of the lander quietly listening to the "pulse of Mars."

During this period, the detectors recorded more than 1,319 earthquakes.

By measuring how fast seismic waves travel, scientists have determined the materials through which seismic waves are most likely to travel.

"These techniques are actually the same ones we use on Earth to explore for water or find oil and gas," explains UC Berkeley Professor Michael Manga, who was involved in the study.

The analysis showed that reservoirs exist about 6 to 12 miles (10 to 20 kilometers) deep in the Martian crust.

"Understanding the Martian water cycle is critical to understanding the evolution of climate, surface and interior," said lead researcher Dr. Vashan Wright of the Scripps Institution of Oceanography at UC San Diego.

Professor Manga added that water is "the most important molecule in shaping planetary evolution". He said the discovery answers the big question "Where did all the water on Mars go?"

Studies of the Martian surface and its channels and ripples indicate that in ancient times there were rivers and lakes on the planet.

But for three billion years, this place has been a desert.

When Mars lost its atmosphere, some of the water was lost to space. But on Earth, "most of our water is underground, and there's no reason why that shouldn't be the case on Mars," Professor Manga said.

The InSight probe can only record directly from the crust beneath its feet, but researchers expect similar reservoirs to exist on Earth. If so, they estimate there is enough liquid water on Mars to form a layer on the surface more than half a mile deep.

However, they point out that the location of Martian groundwater is not good news for billionaires who want to develop plans to colonize Mars.

"It is trapped 10-20 kilometers deep in the Earth's crust," explains Professor Manga.

"Drilling a 10km deep hole on Mars would be difficult even for (Elon) Musk," he told BBC News.

The discovery may also point to another target in the ongoing search for evidence of life on Mars.

"Without liquid water, there is no life," Professor Manga said. "So if there were habitable environments on Mars, they may be deep underground right now."