By Dr Chris Riley BBC Science

Oil geologists have stumbled across an ancient impact crater under the North Sea, 130 kilometres (80 miles) east of England's Yorkshire coast.

The three-kilometre-wide chasm named "Silverpit crater" was punched into the crust 60-65 million years ago, and seems very well preserved 1,000 metres (3,300 feet) beneath the sea bed.

Two Aberdeen-based geologists looking for hydrocarbons in the Trent North Sea gas field were re-examining 3D seismic reflection surveys that were done in 1992 when they made the discovery.

We are as sure as we can be that Silverpit is the UK's first impact crater

Phil Allen, UK geophysicist

Using the very latest seismic interpretation software, geophysicist Phil Allen had been studying deep Carboniferous rocks which form the gas field, when he found a structure that caught his eye higher up in younger rocks.

Despite 30 years of experience he said he had never seen anything like it.

"I thought it could have been caused by an impact," recalls Allen, "but it could equally have been created by another process."

Unable to resolve its origin further, Allen pinned a printout of the mystery structure to the office wall with the caption, "anybody seen anything like this?"

'Eureka' moment

It was six months before he got a reliable answer, when structural geologist Dr Simon Stewart walked into the office to discuss the gas field.

Unbeknown to Allen, Stewart was something of an expert on the geological causes of circular structures and immediately recognised the central peaked cone on Allen's printout as a characteristic of an impact crater.

The first image of the Silverpit crater Courtesy PGL/BP

The 3D profile alone was not enough to confirm that the structure was caused by an asteroid impact, and even two exploration holes drilled through the crater structure in 1984 and 1993 were not enough to establish its catastrophic origin.

Allen and Stewart looked to regional magnetic data and seismic reflections of older rocks beneath the crater to rule out volcanism or a rising plume of salt.

"We are as sure as we can be that Silverpit is the UK's first impact crater," says Stewart.

Judging by the size of the crater and the density of the surrounding rocks, the asteroid or comet that made it probably weighed around two million tonnes and could have been 120 metres (390 feet) across.

It would have impacted into a shallow sea that covered Britain back then and would have sent disastrous tsunami tidal waves surging across the nearby Cretaceous land masses.

'Outer ripples'

The geologists say the crater has been uniquely "fossilised", by sediments settling into the depression, and is unlike any other impact crater so far found on Earth.

Tyre crater on one of Jupiter's icy moons, Europa Courtesy PGL/BP

"Other craters on Earth we know about were created in hard rocks, whereas Silverpit would have been formed in soft underwater sediments - creating a very different shape of crater," explains Stewart.

A tall conical central peak is buried inside a three-kilometre- (two-mile-) diameter crater that is itself surrounded by a series of concentric rings which extend out a further eight kilometres (five miles) in each direction.

"Unparalleled 3D mapping of these concentric features down to a resolution of tens of metres shows that the outer ripples are caused by concentric faults in chalk on the sea floor around the central crater that were probably triggered by the impact," says Stewart.

Its shape and size stand Silverpit apart from other craters in the inner Solar System and its closest relative appears to be the Valhalla impact structure on Jupiter's moon Callisto.

A significant first

"The strength of this find is that because we've got such a clear 3D image of the whole thing we can explore its whole anatomy," says Stewart.

"For the first time ever we've had the opportunity to map the 'skeleton' of a crater - not just its surface expression."

With the growing amount of 3D seismic data being gathered across the world's oceans, the geologists think there's a good chance that Silverpit will be the first of many well-preserved soft rock craters on Earth waiting to be revealed.

Full details of the discovery are revealed in the journal Nature.