Hancock's thesis is that in the thousands of years before recorded history in our modern sense started, there was plenty of room for an advanced and technically adept civilisation.

It only takes three or four thousand years, after all, to go from near scratch to manned flight.

Why is this advanced civilisation no longer with us? Well, there have been plenty of ecological catastrophes over the years, too. Something rather less than the upheaval that wiped out the dinosaurs would produce a very serious setback for an urban civilisation. And there are traces.

At this point, things get a bit controversial. We look at the mysterious inclusion in some very old maps of the outline of Antarctica - not as it is now but as it was before three miles of snow were dropped on it.

We consider sundry ruins, marks in a South American desert and the curious recurrence in widely different places of certain themes, notably an Atlantic myth, and a tall, robed and bearded figure who introduces more advanced technology and agriculture in the places he visits.

There are also some significant numbers that come up with suspicious frequency.

Then we come to Egypt. In particular, we come to the three pyramids that appear on all the postcards, and the Sphinx.

Now I always supposed the pyramids were large tombstones whose dimensions reflected the egos of three particularly megalomaniac pharaohs. This is apparently not the case. The dimensions of the pyramids embody with meticulous accuracy some basic geometrical landmarks like the value of pi, the Golden Section and other such constants.

Their relative position and sizes reflect those of the three stars in the constellation Orion, which are commonly known as Orion's Belt. If their relationship to the Nile is considered to correspond with the relationship of the Belt to the Milky Way (which, of course, changes gradually over the years), you can also work out that the pyramids were constructed in about 10,000 BC, long before civilisation is supposed to have started.

They are, in this interpretation, a sort of enormous 'Kilroy was here', saying to any astronomically literate observer coming upon them aeons later that there were people who knew the courses of the stars and could embody them in massive stone structures of impressive accuracy and intricacy.

Things get more difficult when we come to the Sphinx. This was not originally a sphinx. It started as a straight lion and had a face carved on it by one of the aforementioned megalomaniac pharaohs.

As a lion, says Hancock, it would have been pointing directly when built at the constellation Leo, where the sun was at the time rising on the vernal equinox. This is difficult to accept. Everyone knows that if you look at a constellation, it is impossible to discern the mythical figures attached to it by the ancients.

No prudent builder of monuments, it seems to me, would have constructed one that depended for its effect on Leo still being known as Leo after another 10,000 years. Why a lion then? Well, when the Voyager satellite was being prepared to be the first man-made object to fly out of the solar system, it was fitted with a little plate, designed to communicate with any alien who might find it. This was inscribed with some classical geometrical figures designed to convey a sense of basic civilisation even to a finder who could not decipher the solar cells and other technical paraphernalia.

The other thing on the plate was a picture of us.

This leads to a disturbing conclusion. If the pyramids and Sphinx were originally designed to communicate with successor civilisations, the meaning of the lion is clear.

The whole thing was designed and constructed by a race of intelligent cats. I cannot believe this, but be kind to your cat just in case.