The London Basin stretches from Reading to Harwich and out to the Isle of Dogs, and in the middle lies London itself.

The substructure of the layers below the surface was first charted in the 19th Century when there was a need to draw fresh water to replace the contaminated Thames water for drinking.

In 1850 it the layers were noted as sands and gravels, blue "London" clay, clays and sands and the "Lambeth group" - chalk and flints.

It was this chalk that absorbed the rainwater and was able to be siphoned for the clear drinking water for London.

A fine example of the chalk substructure is found at the Chislehurst Caves - a labyrinth of dark passageways which are actually man-made.

There are over 20 miles of caverns and passageways, dug over a period of 8,000 years.

As a result the caves have many Saxon, Druid and Roman associations. The vast complex is also a maze of ancient mine workings.

Centuries later the caves became famous as an munitions store during World War II.