It probably happened millions of years ago, but it would certainly have been one heck of a blast.

Scientists are reporting a newly discovered impact crater in southern Alberta. The crater is so large that creatures standing 200 kilometres away (on the site of present-day Calgary, for instance) would have received first-degree burns from the explosion that created it.

"It would have been roughly 200 times the energy released in the largest-ever nuclear blast," said Doug Schmitt, a geophysicist at the University of Alberta in Edmonton.

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The circular feature, which measures about nine kilometres across, escaped notice until now because it is mostly buried up to 500 metres below ground near the community of Bow City, Alta., about halfway between Calgary and Medicine Hat. A crater that size would likely be the result of a small asteroid some 300 to 400 metres across slamming into Earth's surface.

The discovery offers a vivid reminder that humanity travels in a cosmic shooting gallery. If confirmed, the crater could help researchers refine their statistical estimates of how often such devastating impacts occur. "This could happen any time," Dr. Schmitt said.

Unlike the moon's surface, which is riddled with craters that have accumulated over billions of years, the Earth shows few signs of extraterrestrial bombardment because of erosion and other processes. To date, 184 craters have been confirmed around the world, including the Sudbury basin, the largest impact feature in Canada, but many more are suspected.

The Bow City formation could add to that count. More evidence, including melted rock or minerals damaged by a high-pressure shock, will likely be needed to establish beyond a doubt that it is an impact crater. Still, experts say the evidence, published last week in the journal Meteoritics and Planetary Science, is pretty convincing.

"It looks to me highly likely it is one," said John Spray, director of the Planetary and Space Centre at the University of New Brunswick, who was not involved in the find.

The suspected crater first showed up in 2009 when Paul Glombick, then a scientist with the Alberta Geological Survey, began exploring the region around Bow City as part of a larger mapping project. While examining data from boreholes in the area, Dr. Glombick noticed that bedrock immediately to the west of Bow City seems to form a bowl-like depression.

The depression has a central peak which is typical of larger impact craters and which form when the Earth's surface rebounds after a sudden strike.

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Dr. Glombick consulted with researchers at the University of Alberta who teamed up with him to study the feature in detail. Based on seismological data and other evidence, they concluded it was an undiscovered crater. "To find something that was previously unknown," Dr. Glombick said, "and to be able to put it on the map is really satisfying."

The group's explorations suggest that the crater has largely been scraped away by glaciers and that the circular trace left in the bedrock represents only the "roots" of the crater, rather than what was once a more complete geologic structure. In future, this could reveal new information about some poorly understood aspects of crater formation, Dr. Spray said.

One key unanswered question is when the crater formed. "A crater is more useful … if one can get an age," said Alan Hildebrand, a planetary scientist and impact expert at the University of Calgary.

The sandstone in which the crater is embedded is thought to be about 73 million years old, but that only means the impact could have happened any time after that.

A much larger impact that occurred roughly 66 million years ago is associated with the extinction of the dinosaurs along with a majority of other species that lived on Earth at that time.