Hollywood directors, take note - geologists have pieced together a cinematic account of a violent impact that gouged out a 90-kilometre-wide crater in the US state of Virginia 35 million years ago. Surprisingly, the impact may have created a new niche for life deep underground.

Hidden by younger sediments, the Chesapeake Bay Impact Structure is among the world's largest and best-preserved craters. Now, Gregory Gohn of the US Geological Survey in Reston, Virginia, and colleagues have drilled nearly 2 km into the basin to reveal its formation.

They believe a 2-km-wide object - hurtling faster than 10 km per second - crashed into 200 to 300 metres of water covering soft sediments and layers of hard rock on North America's continental shelf. "The kinetic energy of the impact is just off the scale by anything humans can relate to," Gohn told New Scientist.

The impact formed a transient crater about 20 km wide and 7 km deep. Within 20 seconds, the shock spread a giant wave of debris - including shattered rocks and water - outwards, where some of it fell into the sea.

The floor of the crater quickly rebounded by about 5 km. "It's a very dynamic situation. Rocks act more like fluids than rocks," Gohn says. Rock melted by the impact seeped down into cracks in the shattered crater floor.

Huge landslide

The transient crater's walls began slumping inwards about 40 seconds after impact, triggering a landslide that sent a "megablock" of granite 275 m thick into the crater.

About 6 minutes after impact, water that surged away from the impact zone rolled back into the steaming hole, carrying ejecta splattered from the crater and sediment torn up from the sea floor. Debris blasted into the air rained back to the surface.

By 10 minutes after the impact, slumping walls had left the crater, which was full of churning water and sediment, about 38 km wide. Another ring of debris settled in the ocean about 40 km from the crater's centre.

The impact probably heated the area to 350 °C, killing off any life.

But the team has found living cells at depths of more than 1.4 km, in regions where rocks appear to have been broken up by the impact or its aftermath. Some rocks were also shocked, which made them more crumbly, says team member Mary Voytek of the USGS.

Underground oasis

Such fragmentation and crumbling "provided larger spaces that organisms could move into," she told New Scientist.

Furthermore, the violence of the event moved carbonaceous material from plant and other forms of life from the surface down to great depths, and flushed oxygenated water, which organisms use to breathe, downwards as well.

"It's almost like mixing in some compost into a garden - the further you work it into the soil, the better you make it for the organisms," Voytek says.

The idea that impacts may not be so harmful to life - and may even provide new havens for life deep underground - may be good news for researchers considering the possibility of life on Mars, whose surface is blasted with deadly levels of UV radiation.

Impacts on Mars might have helped to create refuges for life deep underground, Voytek says: "People are really hoping that when we can dig deeply into Mars, we'll find things."