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CRATERS caused by the impact of asteroids or comets long ago are more common on Earth than you might think. They are just a good deal trickier to locate than those peppering the barren, unchanging surface of the moon. However, finding them can reveal details of prehistoric events that have had a lasting impact, as typified by the crater left behind by the asteroid impact that ended the reign of the dinosaurs.

Geologist is the curator of the meteorite collection at the Natural History Museum Vienna in Austria, but he has a sideline in crater hunting 鈥� a difficult and dangerous pursuit that has taken him to some of the world鈥檚 most remote places and even seen him spend time in jail.

Joshua Howgego: Why do you want to find undiscovered craters?

Ludovic Ferri猫re: What motivates me most of all 鈥� apart from the adventure of travelling to these remote places 鈥� is the possibility of making a new discovery. This is nothing unusual. We humans want to discover new things, we are a curious species.

From a scientific point of view, people get excited about impact craters because we now know that one single asteroid impact can have consequences on the scale of the whole planet, like the Chicxulub impactor, a 12-kilometre-wide asteroid that hit Earth 66 million years ago near where the coast of Mexico is today. It left behind a crater roughly 180 kilometres wide 鈥� one of the largest confirmed impact structures of its kind 鈥� and led to the extinction of 75 per cent of all the plant and animal species on the planet at the time, including all non-avian dinosaurs. It changed the course of evolution on Earth. If it hadn鈥檛 happened, we wouldn鈥檛 be here. So finding new craters on Earth helps us better understand the history of our planet鈥檚 bombardment and the consequences for the environment 鈥� close by or at the planetary scale, as well as in the atmosphere.

Of course, it鈥檚 also interesting to document craters to better understand the physics of these huge impacts, for example, how rocks behave and get ejected from them. There are economic reasons to hunt them, too. Quite a large number of impact craters are associated with resources such as nickel or hydrocarbons, though that is not really my main concern.

Crater hunting covers quite a lot of ground, then, no pun intended.

Yeah. Another important aspect is that craters act as traps for sediments. You have this depression that you can date precisely, from radioactive isotopes, for example, and this then starts to fill with sediments from a known time in the past. There have been a few cases where these craters have formed in places where sediments don鈥檛 otherwise accumulate, making them sources of unique climate records.

One example of this is the El鈥檊ygytgyn crater in north-east Siberia, which was drilled by the International Continental Scientific Drilling Program in 2009. Samples from this area that provide a record of the past climate are very rare, but this crater provided an unbroken record of 2.8 million years of Arctic climate change.

How many craters do we know of on Earth and how many are left to find?

Some colleagues and I are working on reviewing this at the moment and centralising all the data on craters under the umbrella of the . I usually say the number of known craters is 鈥�200 plus a few鈥� because there are a few that we are fairly sure are impact craters, but they are not quite confirmed yet.

On the moon there are more than 1 million impact craters with a diameter greater than 1 kilometre. However, we don鈥檛 expect to find as many as that down here. We have plate tectonics on Earth, and this means most of the oldest rocks and early impact record have been erased. Also, weathering and so on buries craters, so the oldest ones are either not preserved or completely buried, and you need to drill to get to them. I think we have probably already found most of the really big craters. But there may be dozens, if not hundreds, of smaller ones left to find.

How do you look for undiscovered craters?

Most often, I start from lists of possible impact sites. There are a lot of these out there where people have reported circular features they have found. Many of them are obviously volcanoes or whatever. But in a few cases they are good impact crater candidates. I start from there, looking at satellite images, geological maps and other publications to compile all the available information. Very often, these things are in remote places, which is why the data available is quite limited. Then you have to go out and look for the rocks that can prove this is an impact crater.

During an impact, you have a huge release of energy and searing temperatures, which shapes and transforms rocks and minerals, so they are quite different from what you would expect of their original environment.

One thing I look for when in the field is shatter cones, which are rocks with distinctive cone-shaped striations on their surface. We know these are only formed in nature during a meteorite impact, due to the propagation of shock waves that generate this unique fracture pattern. These are the only rocks that, if you identify them in the field, you can say, 鈥淏ingo, I found a new impact crater.鈥� They are exciting to find.

The other thing you do is collect samples and take them back to the lab where you can use a microscope to look for telltale minerals called shocked quartz 鈥� quartz grains containing deformations. I get samples sent to me from people all around the world. Sometimes people who are not necessarily geologists send me samples from a crater they think they have discovered. By characterising the shocked quartz grains, we can not only confirm it is an impact crater, but also do things like estimate the shock pressure [the forces the rock experienced due to the impact] 鈥� we ideally want to be able to describe exactly what happened in these events.

I have just published results about , in Brazil, which has long been a suspected impact crater. My collaborators and I have found shocked quartz grains, which prove that it is indeed an impact crater, the 11th known in the whole of South America.

Can anyone get involved in hunting craters?

There is a French citizen science platform I am involved in called [which translates as 鈥淟ookout for craters鈥漖. We wanted to start from scratch and provide everyone with access to this very specific satellite imagery, so-called shaded relief, which helps you see circular depressions invisible on standard satellite imagery. We have definitely found a few good crater candidates through this method.

I also want to use artificial intelligence for this work. There were a few attempts decades ago, but it didn鈥檛 really work. If you just ask AI to look for a circular feature it finds them everywhere and circles a lot of rubbish; sometimes I think this job would be easier if impact craters were square. The idea now is to train an AI on the citizen science data, which should make it more effective.

What is it like hunting craters out in the field?

In 2013, some colleagues found a possible impact crater on satellite imagery. It was 7 kilometres in diameter in eastern Gabon, right next to the border with the Republic of the Congo. In 2019, my PhD student and I decided to go there to get rock samples. After crossing the whole of Gabon, we reached the proposed impact site and were told that we were actually in Congo. The border is obviously not as clear as it seems on maps and GPS. We did not have permission to be in Congo and were arrested by some police officers. Luckily, before being taken to a jail, I asked if I could take a shower. They let me do that, but I didn鈥檛 really want to take a shower 鈥� I wanted to go down to a river and collect a few samples of rocks and sand.

How did you manage to conceal those in jail?

I just put the samples in with my belongings and they never checked. We were in jail for four days. When we got back, I looked at the sand under a microscope hoping to find some shocked quartz grains. But we didn鈥檛 find anything and so we haven鈥檛 proved that this is an impact crater yet. I need to go back there.

You have had more successful expeditions, right?

The most exciting one was about 13 years ago, when we went to the Democratic Republic of the Congo (DRC). From satellite images, the crater shows up as an amazing, circular, complex feature that is 17 kilometres in diameter. When you see something that big and with such a characteristic morphology, I mean, it cannot be anything else except an impact crater. No one had confirmed it, though, because the DRC is not a place you generally go for holidays. A few weeks before the first time I had planned to go, my permission was withdrawn due to security threats.

But I went the year after, in 2010, with a local geology student. It was in a really remote place; we went by foot for the last 25 kilometres because there was no way to approach the structure with a four-wheeled car. On the second day, we found shatter cones. I have a picture where I鈥檓 smiling because I know at that moment that I have discovered impact crater number 182. I went back again in 2013 to find more rocks that we could use to try to date the crater.

It sounds like quite an adventure.

Later on, I wanted to write my name on a tree in the centre of the crater. I was using my geological hammer to knock the letters into the wood 鈥� and when I struck the tree a snake fell out of the branches above and onto the student standing next to me. Thankfully he was fine.

What have we found out about that crater?

First of all, we have confirmed that it is an impact crater, a rather large one, and still to date the only confirmed impact crater in the whole of central Africa. We have been working to date it and that鈥檚 taken a very long time. We know that the rocks in which the crater formed are at maximum about 575 million years old, so the crater must be younger than that. To be able to put an exact age on a crater is quite hard because you need to find a rock that was completely melted during the impact, which allows atoms to diffuse between the rock and its environment so that its radioactive clock was reset. We are in the process of publishing our latest results on this 鈥� it takes a lot of careful work to be sure that we date the impact and not something else, but we think that we have a good age for it now.