Ever since Andrija Mohorovicic named the Mohorovicic Discontinuity in 1909, it has exerted a powerful fascination on people who think about the deep Earth. Taking a look at what lies beneath would allow us to move the Earth sciences beyond peering at the crust, which is only a minute and unrepresentative sample of the whole planet, to see something much closer to the whole picture.

It is true that mantle rocks such as peridotites bring themselves to the Earth's surface for our delight unaided, and that even basalt, one of the Earth's commonest rocks, is only one step away from being mantle material. But that did not stop Project Mohole, thought up in 1956 by the US National Science Foundation and billed as the Moonshot of geology. It turned out to be more Apollo 13 than a giant leap for mankind.

One bad sign might have been the fact that a group called The American Miscellaneous Society was in charge, a body set up in response to all the unclassifiable science proposals put to the US Navy.

Things started OK with the realization that the thin crust of the oceans was the place to drill. Phase 1 lasted from 1958 to 1966, and resulted in a series of holes a couple of hundred metres deep. The planned Phases 2 and 3 never happened. Later Moholes have been more impressive. One in the Kola Peninsula in Russia ran to over 12km, but still did not get through the crust. It was about 2km deeper than the deepest well so far drilled for commercial purposes, a gas well in Oklahoma.

While the world seems to have moved on from the concept of Mohole drilling, there is still plenty to be learned from deep holes in the Earth. In Germany, a 9km hole has been drilled to find out more about the way fluids move underground and how fault systems develop. It turns out to be surprisingly easy to
simulate a swarm of earthquakes by building up pressure in the well.

In Iceland, much shallower wells drilled in a volcanically active area are also showing research promise. The Iceland Deep Drilling Project exists to extract heat from Iceland's geothermal reserves at a far higher temperature than the steam and hot water that emerge at the surface. The ambition is to drill down to about 5km where there will be superheated steam on tap for electricity generation and other uses.

Finally, 23 organisations from around the world are so convinced about the wisdom of deep drilling that they fund the Integrated Ocean Drilling Program, under which a drilling rig called the Joides Resolution sails every bit of blue on the map to answer questions about the make-up of the Earth's crust by drilling
into it.

Indeed, in April 2005 it turned out that the crew of the Resolution might be about to deliver us a Mohole by stealth. They had already reached the base of the crust and might break through to the mantle some year soon. But have no fear. As we have seen, the mantle is liquid enough to creep along at a rate of a few centimetres a year, pulling the continents along and driving plate tectonics. But it is far too viscous to spurt out of a possible Mohole, volcano-style, especially as the hole would be well under a metre in diameter. Instead, it would be a feat of high technology to get rock samples from the top of the mantle back to the surface to go under geologists' microscopes.