LOST LANDS AND CATASTROPHESINTRODUCTION

On July 16, 1994, a small fragmenting comet known as Shoemaker-Levy began ripping through the atmosphere of the planet Jupiter, causing explosions of almost unimaginable intensity. As the second fragment fell there was a blast equivalent to 250 million tons of TNT--several times more powerful than all the world's nuclear arsenals put together. When the third chunk of the comet struck it created a hole in Jupiter's atmosphere the size of the Earth. The full extent of the damage that Shoemaker-Levy inflicted on Jupiter is still being assessed, though one thing is already perfectly clear: the long-cherished scientific belief that comets are harmless and cannot crash into planets has been dispelled forever.

The question immediately arises--could a comet, or cometary fragment, crash into the Earth? Or has it already done so? In the seventeenth and eighteenth centuries, in the days before Darwin, scientists freely speculated about such matters, wondering whether a comet might have been responsible for the Great Flood described in the Bible. While theologians were happy to accept that the Flood was caused by God directly, scientists were busy researching possible physical mechanisms. Some, including the great Edmond Halley (who gave his name to the famous comet), looked beyond the Earth for a trigger. In 1694 he proposed, in a paper to the Royal Society, that Noah's Flood was caused by the collision between the Earth and a comet, which landed in the Caspian Sea and drenched the surrounding lands with water. Others speculated that a watery comet was responsible.

From the standpoint of pre-Darwinian science, belief in a Great Flood was entirely reasonable, as such an event seemed to explain many of the world's greatest historical enigmas. The rocks that scientists were beginning to examine were full of the fossilized remains of millions of extinct plants and creatures, and a catastrophic flood could account for why these life-forms no longer existed and why their remains had been trapped and preserved in sedimentary rock. It seemed natural, then, to borrow an explanation from the Bible, which told of the deluge in the time of Noah. Assuming that there had been a real Flood also provided an economical explanation for why there are so many similar legends around the world.

Such quaint ideas went completely out of fashion in the early nineteenth century. The relatively new science of geology was maturing, and the naive view that all the world's rock strata had been laid down in a single event was seen to be unworkable. It was becoming clear that there was a whole sequence of layers from different ages, laid one upon the other, each containing its own life-forms. The question now was what had created these strata and how long they took to form. One school of geology was the catastrophists, who expanded the idea of a Great Flood into a whole series of cataclysms--sometimes of water, sometimes of fire (from volcanic activity). Their opponents represented a new school of thought--the uniformitarians. Founded in the 1830s by lawyer Charles Lyell, they set new ground rules for the debate. Geologists generally agreed that special causes--such as direct divine intervention--should be excluded from scientific discussion, and Lyell now introduced his principles of 'uniformity,' which attempted to rule out special events as well.

The law of uniformity states that the 'present is the key to the past': only the same forces that are visible today were responsible for shaping the world. On a general level this makes perfect sense. There is no point assuming that the laws of physics were once different and, for example, that...