Pat it into a round about 5cm (2in) deep and cut a cross on it – to let the fairies out! Let the cuts go over the sides of the bread to make absolutely sure of this.Bake the bread for 15 minutes, then turn down the oven to 400F/200C/Gas 6 for a further 20-30 minutes, or until it is cooked (tap the bottom of the loaf; it should sound hollow when you do so). The mantle extends to a depth of about 2,900km but there are various layers within it that reflect earthquake waves. One, at 420km, represents what is known as a phase change: below it, minerals with the same chemical compositions are forced into a denser crystal lattice structure by pressure. This depth marks the transformation of green olivine into a denser, brown silicate called spinel There is another feature about 670km down That marks the boundary between the upper and lower mantle.

Once again, there is a phase change across it, with the spinel structure above and an even more tightly packed one called perovskite below. Perovskite is probably the most abundant mineral in the Earth. Its tight-packed structure makes it of special interest to scientists looking for very hard materials and new types of super-conductor for the electronics industry. But at surface pressures perovskite is unstable and scientists have to struggle to make it, even in milligram quantities. No further beneath our feet than the horizontal distance between London and Edinburgh, there must exist millions of tons of it.The phase change at 670km represents a more substantial barrier, both to mantle circulation and to seismic waves, than the one at 420km. The path of the descending slab of old, cold ocean crust can be traced from the epicentres of deep earthquakes.

They map its descent down to 670km where it seems to grind to a halt, at least temporarily. One of the biggest controversies in geology is whether, after absorbing sufficient heat to undergo a further phase change, it then continues through the lower mantle so that the entire mantle takes part in the circulation, or whether the mantle is like a great double boiler with separate circulation in the upper and lower mantle and little or no chemical mixing between them.Evidence from simulations is now beginning to suggest a compromise. The phase change from spinel to perovskite takes up a lot of heat, so the descending slab cannot undergo the change and cross the boundary until it has had time to warm up – probably millions of years. So it tends to spread out into a sort of holding reservoir that forms a pronounced layer in the seismic images.