A short story by Arthur C. Clarke, first published in Science-Fantasy Summer 1950 Edition.

The story starts with two geologists, Barton and Davis, and a palaeontologist, Professor Fowler, excavating a trail of dinosaur footprints somewhere in the tropics. They discover that a nearby laboratory houses the two of the world's best physicists - Barnes and Henderson, who specialise in low-temperature physics.

We learn that Henderson is investigating Helium II, a liquid that only exists at a few degrees above absolute zero. Another strange property of Helium II, explains Davis, who happens to have a degree in physics himself, is that is exhibits negative entropy. For the benefit of Barton, Davis also explains that entropy has been described as "Time's Arrow" because, unlike a clock, it only goes in one direction. Thus, realises Barton, means that negative entropy could mean negative time!

Henderson invites Fowler to his lab, but neither we nor Davis and Barton are told what happens there. Davis and Barton investigate the previous work of Henderson and come up with the idea that he is building a machine to see into the past, using negative entropy. They tell him as much when he comes to visit the dig. Henderson indicates that this might be true, and asks Professor Fowler to come to his lab when he tests out his mysterious device.

The dig, meanwhile, has been getting on quite well. They work out from the spacing of the dino prints that the dinosaur in question is speeding up, as if it were chasing something. As Barton and Davis are uncovering a new section of the track, the Professor sets off in the Jeep for Henderson's lab. Some time later, Davis sees the lab explode, and witnesses a "ripple" spread out from the site. He assumes that the experiment has gone terribly wrong. He finds Barton, who has discovered what the dinosaur was chasing:

Fossilised into the stone, almost obliterated by a dinosaur footprint, is the zig-zag print of a Jeep tyre.