European researchers hope to pin down exactly where Mars is hiding its water by looking through the planet rather than on its surface.

A ground penetrating radar system aboard Mars Express, a European Space Agency (ESA) probe set to launch in early June, will use radio waves to map out any water reservoirs as deep as 3 miles (5 kilometers) below the surface.

"We are looking for serious amounts of water, for mountains of the stuff," said Iwan Williams, a professor with Queen Mary University of London and co-investigator of the radar experiment. "Everybody believes that it's there."

Williams told SPACE.com he hopes the radar aboard Mars Express will provide definitive answers as to where to find subsurface water, the existence of which has, to date, only been inferred through satellite observations.

Once in orbit, the 131-foot (40-meter) MARSIS antenna boom will unfurl from Mars Express, a craft with a widest point of 6 feet (1.8 meters). "The antenna folds up like a magician's trick," Williams said. "You just push a button and away it goes."

MARSIS will simultaneously send out low frequency radio waves at two different frequencies to make its radar soundings. Although the majority of these of signals are reflected by the Martian surface, some will make it deep enough to relay information about the planet's interior composition.

A cache of underground water, for example, should reflect signals more strongly than the surrounding rock, and clue researchers in on its location and size. Weaker radar echoes should allow MARSIS to determine the thickness of sand dunes and examine the different layers of Martian rock underground.

"Water is really what you want to see," said Gerhard Schewm, head of the Planetary Missions Division at ESA, in a telephone interview. "And not just water in mineral deposits, but liquid water."

Because the radio frequencies are so low -- between 1.3 and 5.5 megahertz -- researchers plan to make the most of their subsurface observations at night to prevent interference by the Martian atmosphere during the day, when free electrons in Mars' ionosphere will reflect much of MARSIS' radar signals. Researchers also have to wait until Mars Express is within 500 miles (800 kilometers) of the planet to take accurate radar soundings, giving researchers about 26 minutes during each 6.75-hour orbit to look for underground water.

But instead of sitting idly by during the Martian day, MARSIS will bounce signals off the planet's ionosphere to measure the effect of solar wind in the upper atmosphere and attempt to make subsurface observations when possible, researchers said.

The MARSIS experiment is the result of collaboration between ESA and NASA's Jet Propulsion Laboratory in Pasadena, California.

Earlier this year, researchers poring over data from Mars Global Surveyor announced that the planet's ice caps consisted mostly of water, not frozen carbon dioxide as had been previously thought for decades. Global maps generated by Mars Odyssey as it orbited the planet found large amounts of subsurface hydrogen thought to exist as water or ice, so much so that it could cover the planet in ankle-deep water.

"If [water] is not there, it has serious implications for the study of Mars," Williams said, adding that water has always been a cornerstone for many scientists exploring the possibility of life on Mars and eventual human exploration there. "Without it, the planet becomes a much more inhospitable place."

MARSIS should be able to observe most of Mars over the course of one Martian Year, about 687 Earth days, and allow researchers to build up a three-dimensional representation of the planet's upper crust.

"This is a very important instrument," Schwehm said of MARSIS. "It will give us a global view of the Mars subsurface and allow us to see the structure of water distribution there."

The findings from MARSIS, he added, will add to those of the Beagle 2 lander, the orbiting Mars Odyssey and Global surveyor, as well as the upcoming robotic rovers NASA expects to launch in June.

"All these studies together will give us a good picture of the planet," he said.

Ground-penetrating radar isn't a new method of peering below the crust of a planet.

On Earth, the radar is mounted on airplanes and used to find new water sources and oil wells before the first drill bites into the dirt. NASA researchers used the method on the Moon during Apollo 17, probing the lunar underground to determine its composition.

But MARSIS will be the first tool of its kind to search for water on another world, and could put to rest once and for all where Mars keeps its aquatic sweet spots.

"I would love to be able to find the big lakes, or some sort of frozen lake hundreds of meters down below the [Martian] surface," Williams said. "Most people believe water is there, and I think the failure to find it will more surprising than success."