Mars might have been a wilder water world than we thought. Scientists scanning the terrain have discovered signs of two massive tsunamis in the northern hemisphere — events that might have been caused by massive, crater-carving impacts.

The findings, described in Scientific Reports, bolster the idea that Mars once held an enormous ocean and depict a Red Planet that was far more dynamic than previously expected.

While Mars may look dry and dead today, it seems to have been much more Earth-like in the distant past, featuring a thick atmosphere and large bodies of standing water. It’s thought that our arid neighbor may have actually had a major ocean about 3.4 billion years ago, although that idea hasn’t been universally embraced — in part because scientists haven’t been able to find a steady, continuous boundary in the terrain that would have demarcated that ancient body of water.

“What a lot of people were doing in the past was to look for shorelines distributed along constant elevations, which is what we see here on Earth,” said lead author J. Alexis P. Rodriguez, a senior scientist at the Planetary Science Institute in Tucson, Ariz. “However, we didn’t find it and that kind of remained a paradox. Why do we have an ocean but we have no constant shoreline around the ocean?”

Instead, several years ago, Rodriguez decided to look instead for signs of tsunamis — and he found them, in the form of lobe-shaped, boulder-rich deposits of debris in northern Mars. These lobes had been recognized before, but their significance had remained unsettled.

Most of the time, when you have such lobes of material, they indicate that water was pushing debris forward. Since water flows from high to low ground, the lobes usually point downward. These ones, however, pointed up, against gravity’s pull.

Aside from tsunamis, “there’s not too many other processes that can form lobate, landward deposits,” said study co-author Virginia Gulick, a planetary scientist at NASA Ames Research Center. “A tsunami deposit is like a flood deposit; it’s just in the reverse direction.”

Using data from the Mars Odyssey thermal imager, the Mars Reconnaissance Orbiter’s context camera and the Mars Global Surveyor’s laser altimeter, the researchers were able to assemble a map of certain regions in the northern plains that allowed them to analyze how these strange shapes came to be.

They found that two massive tsunamis raged through the area, probably within a few million years of each other; the first washed over an area of about 800,000 kilometers and the second swamped an area of roughly 1 million square kilometers. The typical wave height as it hit the shore would have likely been about 50 meters but, at various locations, could have ranged anywhere from 10 to 120 meters or so. That’s far taller than the wave that hit Japan after the 2011 earthquake, which rose an estimated 39 meters.

What could have caused such a massive wave? The scientists have a guess: space-borne missiles such as asteroids and comets smashing into the planet, leaving a crater and triggering these moving mountains of water. The kind of impact that would leave a crater with a diameter of at least 30 kilometers could do the trick — and the scientists said they found about 23 such “marine impact craters” in this part of Mars.

Of course, it’s just a hypothesis for the moment, because they can’t match a particular crater to one of the events. But it makes sense, given that similar things have happened on Earth: The asteroid that killed the dinosaurs some 66 million years ago launched a massive tsunami as well.

For those wondering if there was ever life beneath the surface of ancient Mars, perhaps these walls of liquid destruction unearthed some of that old evidence, Gulick pointed out.

“If there was life that formed, say, in the large body of water,” she said, “then tsunami deposits might have brought up some of that material that might show evidence for microbial activity in the past.” (Whether it exists at all and survives to this day is another matter entirely, of course.)

The findings also explain why scientists can’t seem to find continuous shorelines: The tsunamis totally buried them. The new evidence bolsters the case for this giant body of water and paints a picture of a more active Mars than expected.

“To see these types of events that we’re familiar with on Earth and to see similar features on Mars is really exciting,” said Lisa Ely, a geologist at Central Washington University who studies tsunamis on Earth and was not involved in the paper. “There’s no reason to think Earth is the only dynamic planet out there.”