Aaaargh a giant hole!

This morning a hole has suddenly appeared in the middle of the drive.
On the surface it is not huge, but when you look down it you can see that under the ground it is much bigger. Perhaps a couple of meters square and 30cm deep. We wondered if all the rain had somehow washed away the soil under under the hard surface of the drive, but somehow it doesn't really add up. You would think it would look damp if it has all been washed away plus it would be strange it has washed away only in the middle of the drive only and in quite a neat shape. Another idea is that there was some kind of pipe or tunnel under there to drain water away that has collaped. You would perhaps expect to see some remains of the pipe which we have not yet.

Less sensible ideas that have been proposed are, one of the meteors hit it, a giant mole dug it out, an army of shrews marshalled by Pernod dug it out...

We are not quite sure what to do about it, but need to do whatever it is fairly quickly before the car falls in and is swallowed. It is going to be hard to fill in without knocking out the rest of the top layer which would make even more of a disaster of the drive than it already is. We are thinking our initial approach will be to try to fill it in by putting rocks and gravel down there and using something to push them under the existing surface of the drive, I imagine the existing surface would then pack down on to them giving a lovely dip in the drive.