Water could be trapped deep inside the crust of Mars.
NASA scientists have discovered a reservoir of liquid water on the Red Planet that has increased hope of discovering other forms of life there.
Analysis of data from the US space agency's Mars Insight Lander revealed "seismic signals" of liquid water from deep within the outer crust.
Although there is water frozen at the Martian poles, this is the first time that liquid water has been found on Earth's neighbour.
Water is an essential element for any form of life and the team admits that their findings give cause for optimism regarding extraterrestrial discoveries.
Dr. Vashan Wright, co-author of the research at the University of California San Diego, said: "The presence of water does not signify that there is life, but water is thought to be an important ingredient for life.
"We know that life can exist in the deep subsurface of the Earth, where there is water. The mid-crust of Mars at least contains a key ingredient for habitability and life as we know it."