What is seen as a large impact crater has been identified under the Greenland ice sheet.
The 31km-long depression was revealed when scientists examined the island's rock bed radar images.
Investigations show that the feature might have been dug up by iron asteroids along 1.5km around 12,000 and three million years ago.
But without drilling through nearly 1 km of ice for direct bed samples, scientists cannot be more specific.
"We will try to do this; it will definitely be the best way to get the dead fish on the table, so to speak," Prof. Kurt Kjaer, from the Danish Natural History Museum, told BBC News.
These findings are reported in the journal Science Advances.
- How Greenland burned the bottom
- How Greenland will look without the ice
What is the crater like?
The putative impact crater is located right on the northwest edge of the Greenland Ice Sheet, below what is known as Hiawatha Glacier.
The additional high resolution radar imagery collected by Prof Kjær's team clearly shows an increased circular structure on the rim and at its center – both are classic features. But because the depression is covered up to 980 m ice, scientists have so far had to rely on indirect studies.
What is the supporting evidence?
Meltwaters ran out from under the Hiawatha Glacier to Nares Strait carrying sediments from depression. In these sediments are quartz granules that have experienced tremendous shock pressure, of the type that will be experienced in an impact.
Other river sediments have revealed unusual ratios in different metal concentrations.
"The profiles we see are rhodium enrichment, platinum depletion, and palladium enrichment," explained team member Dr Iain McDonald, from Cardiff University, UK.
"We are very excited about this because we realize we don't see rocky meteorites, but iron meteorites – and not just old metal meteorites; it must be a very unusual composition."
Signatures close – but not the same as – pieces of meteorite iron collected on Cape York not far from the Hiawatha site. This is unimaginable, the team argues, that Cape York material represents a piece that came from the body of the main asteroid when it fell to Earth.
What are the doubts?
One concern is that there is no trace of impact in some cores that have been drilled through the ice sheet in the south. At the very least, this might be expected to combine dust falling from the sky after the incident.
The other head of the rake is the absence around the location of Hiawatha from any rocky material that will be ejected out of the crater during a collision.
Prof Kjær said this missing signature might be explained by a very shallow angle of impact which took most of the ejecta to the north. And if the area is covered with ice, the debris might then be removed.
"We know that at one time Greenland Ice Sheet joined the Canadian Ice Layer, and flowed into the Nares Strait. If you want to find this material today, you have to do deep drilling in the ocean," Prof. Kjær explained.
What is the age limit?
The team knew that the crater must be older than about 12,000 years because the undisturbed ice sheet above depression can be lined with layers of core drilling that have been dated directly.
And they estimate a age younger than three million years based on an assessment of the possible level of rock erosion, both in the crater and in the nearby terrain. But the only way to get a definite age for the crater is to drill and collect stones for laboratory calendars.
How is this connected with other ideas?
If the impact is right at the end of the window age it will definitely rekindle interest in what is called the hypothesis of the impact of the Young Dryas.
The Younger Dryas is a period of strong cooling amid climate warming that occurs when the Earth emerges from the height of the last ice age.
Some people argue that the impact of asteroids might be responsible for this cooling change – and the extinction of various groups of animals that occurred at the same time throughout North America.
Others, though, have been critical of the hypothesis, not least because there is no crater that can be associated with such an event. Hiawatha Depression now tends to blur the embers that are dying from this old debate.
Dr. Mathieu Morlighem, member of the team from the University of California, Irvine, USA, commented: "When you think about it, beds under the ice sheet must have craters that have not been explored, and maybe even some in Antarctica too, but radar measurements are needed to place it, and post it is very challenging. "