I never would have recognized the mile-wide Versailles structure had Thompson not pointed it out. For one thing, there's no obvious depression. The circular outline of the crater is marked only intermittently by small, deep sinkholes in the limestone terrain. The impact, if one occurred, happened as long as 440 million years ago. Versailles was discovered in 1962 and was originally thought to be a giant sinkhole itself. But later geologic mapping showed faults around the perimeter similar to those seen around impact craters. And the surrounding fields yielded a surprising number of breccias, lumpy conglomerate rocks made of sharp rock fragments fused together. Geologists often find breccias near volcanoes, but there aren't any volcanoes around here.
By the roadside, across a barbed-wire fence, Thompson spies what he thinks might be a breccia. "I wouldn't mind having a piece of that for myself," he says, then decides against it. We haven't gotten permission from the landowner, and he can get a piece when he comes back. He'll need to do more extensive fieldwork if he wants to prove Versailles a crater.
Kentucky's other candidate, Jeptha Knob, is 24 miles away. A three-mile-wide circular formation with a clump of hills sticking up from its center, it was one of the original "cryptovolcanic" structures that puzzled Walter Bucher back in the 1920s. Geologic maps reveal a neatly circular pattern of faults at the perimeter, where Thompson shows me contorted rock beds, evidence of the impact that occurred some 440 million years ago, back when Kentucky was under a warm, Caribbean-like sea. This was long before the dinosaurs, so there were no large animals to look up at the screaming fireball. Just a bunch of shelled creatures whose fossilized corpses I can easily pick out of the crumbly Ordovician-era breccia with my finger.
We head toward the center of the crater, which is on land owned by Cal Schmidt, a genial, soft-voiced man who looks to be in his early 70s. Schmidt greets us at his home on the edge of a small private lake. He's obviously proud of owning the central uplift feature of a suspected impact crater. When I tell him I spent the day before at Middlesboro, he quickly turns to Thompson and asks, "Is Middlesboro bigger than me?" A joke, but with a hint of concern.
Over lunch, he gives us the recent-at least in geologic terms-history of Jeptha Knob, how his dad bought it for $26 an acre in 1926, how the bandit Frank James once spent a winter hiding there, how the name comes from an Old Testament warrior. Later, the three of us walk up onto the wooded hill behind the house, where Thompson is on the lookout for breccia deposits that another geologist mapped here many years ago. Shocked quartz is unlikely to turn up at this particular site because it was carbonate rocks, not quartz sandstone, that got smashed in the long-ago impact. Bad luck. But Thompson is studying a core sample, a small amount of material only two inches in diameter but drawn from as deep as 2,000 feet, to find out more about the geology below the surface. He plans to compare the characteristics of the sample with those of core samples from confirmed impact structures. In doing so, he may discover evidence that will place Jeptha Knob on The List, right there between Jänisjärvi in Russia and Kaalijärv in Estonia.
Thompson has had to dig a little deeper than geologist Kevin Evans of Southwest Missouri State University in Springfield. Evans recently discovered a 12-mile-wide crater, the Weaubleau-Osceola structure in Missouri, that, if confirmed, would be one of the biggest impact craters in the United States. It's not on The List yet, but that's just a matter of publishing the scientific paper, thinks Evans. He's already found shocked quartz, breccias, the whole bit.
Not to mention a huge bull's eye that popped up on his computer one day.
Weaubleau had been suspected as an impact site because of the tortured rocks in the region, but no crater was obvious. Evans was using a computer graphics program to stitch together four U.S Geological Survey digital images of the area when a thumbnail composite image came up on his screen. "Boom, there was a big circle," he recalls. Nobody had noticed it before because the ring appears broken and happens to straddle the boundaries of the four USGS quadrangles.
Since then Evans has used what promises to become another powerful tool for crater hunting, digital images from the Shuttle Radar Topography Mission, a NASA-Pentagon collaboration that four years ago mapped elevations over the entire globe to an accuracy of about 30 yards, using a radar imager mounted on the space shuttle. When Evans looked at the mission's pictures of Weaubleau-Osceola, another ring only four and a half miles wide appeared inside the circular drainage basin he'd identified from the USGS composite. That, he believes, is the true ground zero.
Remote sensing tools like these might speed up the search for previously unknown craters, particularly ones that are buried or whose surface "expression" is subtle. Other methods have already been applied to that task, including gravity surveys that sometimes show the comparatively loose rubble excavated from a crater as a gravity "low" against a background of solid rock. Using gravity and magnetic maps, Pradeep Talwani of the University of South Carolina last year reported a suspected crater 500 yards under Johnsonville, South Carolina, not far from Interstate 95. If confirmed, it would be the first crater found along the southeast Atlantic coast. There may be other coastal impact scars, similarly buried by sediments.