By Jonathan Amos
BBC News Online science staff
You would not notice it on the ground but from space, the outline is unmistakeable.
The ring that cuts across the Yucatan Peninsula pictured in this space shuttle image is one of the very few pieces of surface evidence that survives from a cataclysmic day in Earth history.
The 3-5-metre-deep and 3-5-kilometre-wide trough (see enlarged photo) traces weaknesses in the rock created by the space impactor that many scientists now believe wiped the dinosaurs from the face of the planet.
The so-called Chicxulub crater - or rather what remains of it; it has since been filled in by limestone sediments - has become the focus of intense study in recent years.
Earth scientists have drilled samples through the crater area and are now investigating the rock chemistry.
They hope to get a better picture of what happened on that day 65 million years ago and discover more about the subsequent catastrophic environmental changes that killed off 70% of all species.
The trough is visible today because of instabilities in the limestone sediments that overlie the crater.
"Even though the crater has been covered up, when there is crustal movement this is accommodated along faults - and we see it," said Dr Joanna Morgan, from Imperial College London, UK.
"But it's a very weak signal. The original offset would have been about a kilometre or so. Today it is just five metres."
The impactor theory is one of several candidates for the demise of the dinosaurs
Dr Morgan is a lead researcher on the international team that is studying the Chicxulub cores. She is looking at the properties of the rock to help determine what the original crater would have looked like in three dimensions.
The collapse of numerous limestone caverns above the crater rim has resulted in an arcing chain of sinkholes - also visible in the shuttle radar image.
"They are the result of extensive erosion in the limestone. These beautiful cenotes, as they are called, are several metres across and were in fact used by the Mayans to make their sacrifices.
"Inside the crater the cenote population is less dense than it is outside. Exactly why the crater (which is buried a few hundred metres below surface) still has an effect on water flow is not clear."
Exactly how the impact caused one of Earth's great mass extinctions is still to be explained, but the billions of tonnes of rock vaporised in the moment of the collision would have produced dramatic global climate change.
"Part of our aim is to quantify the amount of carbon and sulphur released into the atmosphere and then to understand what the environmental disaster was and why it may have caused the extinctions."
Billions of measurements
The Chicxulub image is part of the latest dataset to be released from the US space agency's (Nasa) Shuttle Radar Topography Mission flown in 2000.
The orbiter made 3D measurements of more than 80% of Earth's landmass during an 11-day flight.
CHICXULUB IMPACT CRATER
Approximately 180 km across
Now buried under one km of carbonate sediments
Asteroid or comet responsible for Chicxulub was roughly 10 km in diameter
This involved pushing radar equipment out from its payload bay on an extending boom, one of the largest structures ever deployed in space.
In February, Nasa finished processing the mission's data and delivered it to the National Imagery and Mapping Agency. More than eight terabytes of data have been refined into 200 billion research-quality measurements of the Earth's landforms.
Some further processing will be done before the entire package is released commercially.