On June 26, 1611, one of the world's greatest explorers was having a really bad day. Henry Hudson's ship, Discovery, had spent the previous year mapping what is today northern Canada, but after a long winter spent trapped in ice, the men just wanted to go home. When Hudson insisted they press on to the west in hopes of finding passage to the Pacific, the crew mutinied. They marooned Hudson and a few of his more loyal crewmembers in an open boat, and sailed home to England. Hudson's remains have never been found, and it would probably be small consolation to him that the bay where he was set adrift to die is now named for him. On the shores of what we now call Hudson Bay, Discovery was also the first ship to map an anomaly that we still can't explain today: the Nastapoka Arc.

Nastapoka Arc is a near-perfect circle.

The southernmost projection of Hudson Bay, where Henry Hudson and company spent their last winter, is called James Bay, and reaches south to the top of the Ontario-Quebec border. But look just north of James Bay: the coast of northern Quebec forms a weirdly smooth curve there, fully 155 degrees of an imaginary circle about the size of Wisconsin. Canada, what is going on up there?

It came from outer space…

Since the 1950s, scientists have wondered if the eerily perfect curve of Nastapoka Arc could be a crater from an ancient meteorite impact. It's certainly reminiscent of the round craters we see on the moon, where airless conditions preserve meteor strikes much better than on Earth. According to this theory, the Belcher Islands, a squiggle of archipelago at the center of the arc, would be what's left of the crater's central peak.

…Or did it?

But in 1972, two American geophysicists traveled to Hudson Bay to investigate the crater theory. They and subsequent expeditions have been surprised to find none of the signs a prehistoric meteor strike would have left in Hudson Bay: no shatter cones, no melts, no fractures, no magnetic anomalies, no gravitational anomalies, nothing. If the Nastapoka Arc is a crater, it's deep in the closet.

Time is a flat circle.

So what could produce such uncanny geometry, besides a rock from space? Maybe it's an ancient depression caused by heavy glaciers. Most scientists, however, think the Nastapoka is an arcuate (bow-shaped) tectonic boundary, formed when one shelf of rock was pushed down under another one. If so, it would be the only one on Earth so round that it looks like it was made punched out a cookie cutter 280 miles across.