A vast ocean once covered a third of Mars, scientists believe.
Such a stunning prospect greatly increases the chances of life having existed on the Red Planet, the fourth from the Sun in our solar system.
Researchers have come to the conclusion after using new software to analyse images of the surface. As a result, they have managed to find dozens of valleys to build up the most detailed map to date.

"The areas marked in red show where scientists believe valleys were caused by a network of rivers feeding into the ocean. This area is twice that mapped by earlier research" (bottom picture):

The valleys, first spotted in 1971, were caused by a network of rivers more than twice as extensive as previously mapped. The water channels were in a belt between the equator and mid-southern latitudes.
The experts from Northern Illinois University and Nasa believe they mark the paths of rivers that once flowed from the planet's southern highlands into a huge ocean in the north.

"The area in blue shows where the ocean would have been. The yellow, red and green belt below it is where scientists found the valleys. They believe these were caused by water running from the south towards the ocean in the north":

University of Arizona) – An international team of scientists who analyzed data from the Gamma Ray Spectrometer onboard NASA’s Mars Odyssey reports new evidence for the controversial idea that oceans once covered about a third of ancient Mars.

“We compared Gamma Ray Spectrometer data on potassium, thorium and iron above and below a shoreline believed to mark an ancient ocean that covered a third of Mars’ surface, and an inner shoreline believed to mark a younger, smaller ocean,” said University of Arizona planetary geologist James M. Dohm, who led the international investigation.

“Our investigation posed the question, Might we see a greater concentration of these elements within the ancient shorelines because water and rock containing the elements moved from the highlands to the lowlands, where they eventually ponded as large water bodies?” Dohm said.

"This illustration shows the location of theTharsis volcanic region and Valles Marineris in the context of the hypothesized larger, ancient ocean and smaller, more recent ocean in Mars’ northern lowland planes. Victor Baker and others have long argued that Tharsis volcanism unleashed great floods that carved large outflow channels and deposited sediment carried from the southern cratered highlands to the northern lowland plains, where water formed lakes and oceans and changed climate for thousands of years":

Originally posted by technical difficulties
reply to post by Arbitrageur

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And who knows? There could still be some life there.

Originally posted by Solar.Absolution
S&F! The only thing I don't get... Scientists keep claiming that there were massive oceans / rain etc... But where did all of that water go? It can't just float away into space or absorb into the ground (Because I'm pretty sure earth would be toast)

Originally posted by technical difficulties
reply to post by Arbitrageur

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And who knows? There could still be some life there.

How do we know the meteorite came from Mars?

Meteorite ALH84001 is a softball-sized igneous rock weighing 1.9 kilograms (4.2 pounds). It is one of twelve meteorites discovered on Earth which are thought to be from Mars. Most meteorites formed early in the history of the solar system, some 4.6 billion years ago. Eleven of the twelve martian meteorites have ages less than 1.3 billion years, ALH84001 at 4.5 billion years old being the only exception. All twelve are igneous rocks crystallized from molten magma in a way which suggests they formed in a planetary-sized body, not an asteroid. They have similar oxygen isotope characteristics to each other and higher concentrations of ferric iron, water, and other volatiles than other meteorites. All twelve also show evidence of shock heating, presumably as a result of the impact which ejected them into space. Gas bubbles trapped in one meteorite, EETA79001, have a composition which matches the current martian atmosphere as measured by the Viking Landers, compelling evidence that this meteorite and by association the others, including ALH84001, came from Mars.

After Earth, Mars is the planet with the most hospitable climate in the solar system. So hospitable that it may once have harbored primitive, bacteria-like life. Outflow channels and other geologic features provide ample evidence that billions of years ago liquid water flowed on the surface of Mars. Although liquid water may still exist deep below the surface of Mars, currently the temperature is too low and the atmosphere too thin for liquid water to exist at the surface.
What caused the change in Mars' climate? Were the conditions necessary for life to originate ever present on Mars? Could there be bacteria in the subsurface alive today? These are the questions that lead us to explore Mars. The climate of Mars has obviously cooled dramatically. By studying the reasons for climate change on Mars, which lacks the complications of oceans, a biosphere, and industrial contaminants, we may begin to understand the forces driving climate change on Earth. As we begin to explore the universe and search for planets in other solar systems, we must first ask the question 'Did life occur on another planet in our own solar system?' and 'What are the minimal conditions necessary for the formation of life?'