There's no doubt in most scientists' minds that Mars had liquid water on its surface. Not only that, it had huge rivers, lakes or even oceans. And, as it turns out, even massive floods.
NASA's Mars Reconnaissance Orbiter has been circling our red neighbor for a few years now, taking photos, scanning the surface and even below the surface as much as possible.
Using a radar instrument dubbed SHARAD, the Shallow Radar, and the Mars Orbiter Laser Altimeter (MOLA), researchers were able to peek below the surface of the planet.
In a region near the equator dubbed the Marte Vallis, in the Elysium Planitia plains, they found evidence of an ancient mega flood buried by later volcanic activity.
But it's not as ancient as may be expected, the scientists believe the flood happened some time in the last 500 million years, possibly as recently as 10 million years ago, when Mars was already supposed to be a cold and dry place, much like it is today.
To find evidence of any water this late in the planet's evolution is a major discovery, but the scale of the event suggests that massive quantities of water were responsible.
A volcanic or tectonic event likely triggered the flood and the water went on to wash away soil and erode much of the surface features over a wide area.
The Marte Vallis channel system is some 1,000 kilometers (620 miles) long and is thought to have been formed via water activity. However, much of it is now buried beneath volcanic rock.
This region had been studied before, but the new data provide a much better description of the event and the extent of its effects.
Because the volcanic rock has a different reflection signature than the bedrock beneath, researchers were able to construct a 3D image of the channels below.
From the data, they were able to determine that the flood went through two phases. In the first, water created several channels and flowed around at least four big streamlined islands. The water dug some 40 meters (130 feet) at this stage.
In the second phase, water went slightly to the side, but also dug deeper, suggesting that this phase was bigger than the first.
The rock was eroded to a depth of at least 70 meters (230), but it could be as much as 115 meters (377 feet) since at this depth the radar encountered a different rock layer which is not as reflective as the one above.
The water spilled out over an area about 40 kilometers (25 miles) wide, the entire system is some 100 kilometers (62 miles) wide. It continued to flow and shape the rock beneath for some 1,000 kilometers (620 miles).
With the new data, the source of all this water has been traced to the Cerberus Fossae fracture. This fracture allowed the water in an underground reservoir to reach the surface and massive quantities would have spilled out over a very short period of time, weeks or months.
The researchers have a few theories on what happened to all that water, some of it might have evaporated and later fell as precipitation at the poles. Pockets of water may have simply remained on the surface and froze.
The most interesting possibility is that much of that water found its way underground and has accumulated in another reservoir.
The findings are surprising and incredibly interesting for at least a couple of reasons. There have been some evidence of water accumulating beneath the surface and of large reservoirs deep below even today, but this shows that huge quantities of water build up and can emerge on the surface violently, even in relatively recent times.
This leads to the second big discovery, geological activity at a stage when Mars was thought to have been dormant for at least three billion years. A volcanic eruption or an earthquake triggered the flood, suggesting that the planet is still active, or has been up until recently.