The digger had fallen into a mill pond, buried with its mill more than two centuries ago

A few sweet showers have laid the dust, but they have done nothing to dissolve the drought, which still holds the land in a grip of iron. With a hosepipe ban in force, and official mutterings about the likelihood of long-term water shortages, the temptation grows daily to look for a source of one's own underground.

Some years ago I made contact with a dowser who advertised that he could find water merely from studying a map. Fascinated but sceptical, I sent him a section of the 25-inches-to-the-mile Ordnance Survey. Back it came with four subterranean watercourses marked on it, A, B, C and D. Easily the best, he reported, was B, which was 70 feet down, flowing at 3,000 gallons per hour, and drinkable water.

Amazed by the precision of his findings, I called him in for a site visit, and a few passes with swinging rods confirmed his original diagnosis. The water was definitely there, he said; what was more, it was in a fissure, under pressure, and would rise to within 20 feet of the surface if tapped.

As he was a serious, dedicated man, I have never doubted that he was right. Equally, I have never done anything about the underground stream, because my inquiry was essentially frivolous: I had ideas about making a small lake, but have never got round to it. This scorching summer, however, has revived my interest.

The indications from all over England are that ground water has weathered the drought much better than surface supplies. For instance in the Chilterns, which have huge aquifers hundreds of feet down in the chalk, there has been no hosepipe ban, and the veteran hydrological contractor Jack Hatt (motto: "Dam and Blast") reckons that the vast natural sponges were filled to capacity by last winter's rains. During the summer a farmer summoned him to open a well which had been sealed for 60 years. A tape measure showed a drop of 128 feet to the surface of the water, and 148 feet to the bottom of the well. The water turned out to be clear as gin, and after 900 gallons had been pumped out the level had fallen by only 18 inches.

I suspect that the same applies to many areas of the Cotswolds, which harbour vast aquifers in the underlying oolitic limestone. In that case, what price my own supply?

Any individual householder is allowed to abstract up to 20 cubic metres of water a day - 4,400 gallons - for domestic use, without any licence. Alas, recourse to drilling firms shows that they take a dim view of dowsers, and pay little attention to their findings. Some are brilliant, they agree, but many are unintentionally misleading. Professional drillers such as the Hertfordshire firm Smith & Webb prefer to rely on detailed geographical information - and this, for the moment, I do not have. Besides, to sink a borehole would probably cost at least pounds 3,000. Nevertheless, if I felt convinced that global warming was a fact, I should feel strongly inclined to go ahead.

Meanwhile, farmers round us have been taking advantage of the rock-like ground to drain areas that are normally bogs all year round. In most places this has proved simple, for the surface has been as hard as brick, and heavy machines have been able to move over it with ease. Yet at one point, fully 700 feet above sea-level, the earth sprang a major surprise.

There, only a couple of hundred yards below its source, a little stream flows through the corner of a wood. A contractor had been brought in to clear some bushes from the sides of the watercourse, and, as he worked, he sudddenly found his digger sinking beneath him. So fast did it drop, so deep did it go, that he had to step sharply upwards out of the driving seat and jump for dry land.

The recovery of the digger was in itself an epic; and during the struggle amazing relics came up out of the mud. Among them were roofing-slates and huge coign stones - the corners of walls - which showed that the machine had fallen into an ancient mill pond which had a building or buildings alongside. Not only is there no echo of this mill in living memory, it is not even marked on the tithe map of 1777. It must have been buried under the mud more than two centuries ago.

What sank it? Maybe there was a deluge of rain and a colossal flood. We shall never know. One positively expects the bones of prehistoric foundations to show up when the ground becomes really baked - but it took a one-in- a-million chance to bring our old mill to light.