The crater may have multiple concentric rings, said Brookfield, as is the case in the recently discovered Vredefort Crater of South Africa, the famous Chicxulub Crater of the Yucatan, and on large craters on the moon and elsewhere in the solar system.
If so, there could be evidence in the form of minerals that have been altered by sudden, violent fault movements. There also ought to be jumbles of older, broken rocks called breccia.
Finding evidence of such rings would no doubt bolster the theory that an impact crater exists in Hudson Bay. But just what causes an impact crater to have multiple rings is not exactly clear, said Jay Melosh, a crater researcher at the University of Arizona's Lunar and Planetary Laboratory.
"The moon has multi-ring craters and Mercury has none," observed Melosh. The reason may be that the moon has a more rigid crust with a weak zone underneath.
A multi-ringed crater on Earth would indicate that the impact was felt as deep as the upper mantle — the softer rock zone below the crust. The moon's ringed craters could be from billions of years ago when the moon still had a hot, soft interior.
Jupiter's icy moons also have an abundance of multi-ringed craters and are suspected to have soft interiors under hard crusts, said Melosh.
As for the chances that Hudson Bay holds the largest impact crater on Earth, Melosh said the definitive evidence would be "shocked" minerals that prove extreme pressures, far beyond what any volcano can produce. But without such evidence, he said, it's far-fetched to suspect a crater in Hudson Bay.
Still, "it wouldn't be surprising if there were large impact craters on Earth that we don't know about," said Melosh. That's because the moon, right next door, retains many large craters because it has no weather or plate tectonics to erase them. Based on the moon, said Melosh, Earth should have 6,000 craters larger than 300 miles (500 km) in diameter.
"There's got to be some evidence of the big ones on Earth," said Melosh. "But so far there's no proof."
