The reserve of water that is thought about least often is the one you cannot see. Billions of tonnes of the stuff exists just below our feet and has a vital role in the Earths living and non-living systems.
As we have seen, the Earth gets steadily hotter with depth, and soon reaches the boiling point of water at sea level, 100°C. But as you get deeper, the pressure grows as well, and higher pressure means a higher boiling point. This allows water to exist in the Earths crust at temperatures above 100°C.
But this cannot go on for ever. Water has a “critical point” at 374°C beyond which it cannot stay liquid and is always a vapour. This means that water only exists inside the Earth at comparatively modest depths. There are no secret underground lakes many kilometres below your feet. Many of the solid minerals that make up rocks contain water as a part of their chemical composition. This water is locked away stably in their crystals and is inaccessible unless you take the rock and heat it to destruction. But much of the water within the Earth is “available” water that is mobile and active.
The interior of the Earth is as full a participant in the hydrological cycle as the seas, clouds and rivers. But when you dig into the ground, you don't usually find water, at least not to begin with. The key concept here is the water table.
The water table is the top of the underground zone that is saturated in water. This zone is called the aquifer, a term that comes from the Latin for water-bearing. But unlike most tables, the water table is not a flat, stable structure. It rises and falls roughly in sympathy with the land surface above it. Otherwise everyone who lives in a valley would be paddling while mountain-dwellers would need to drill wells thousands of metres deep to get a drink. You could regard a lake or marsh as a place where the water table breaks out above ground level.
But it would be wrong to think of the zone below the water table as a kind of subterranean water tank. It consists of rock or, if the water table is closer to the surface, unconsolidated sediment, with water in the pores between the grains. Some rocks are better than others at soaking up fluids. Limestones and sandstones are better than most other sedimentary rocks, and sedimentary rocks are preferable to igneous ones. People hunting for large bodies of underground water look for such permeable rocks, as do prospectors for oil and gas, which occupy the same intergranular pores as water.
Water gets into the aquifer via “recharge” areas. Often these are wooded or vegetated regions. Water falling on rocky hillsides tends to be carried off into streams and misses joining the aquifer. Human development of land into cities makes things worse. When a site is developed, surface water is usually captured and sent into rivers down concrete channels and pipes. This cuts down the amount of water getting to the water table. Both this pressure and the growing human demand for water means that the water table is on the retreat in many parts of the world. At the time of writing, some of the most dramatic such effects are being seen in China. They result from the country's rapid economic growth and its population's increased enthusiasm for Western-style food and drink, which require more water-intensive crops.
However, the water table can rise as well as fall. People and industry in London used local wells as their water supply for many centuries. Now they get it from far-distant sources, and little water-intensive manufacturing goes on in the city centre. As a result the water table is now rising back to the level it must have occupied in pre-industrial times, and threatening to dampen deep basements as it does so.
In less habitable parts of the world, underground water takes on an even more vital importance. The Sahara Desert covers nearly 9 million square kilometres and – surprisingly – supports a population of 2.5 million people. They congregate in places where water can be found. The oases that make life possible there are almost all in low-lying areas where the water table is closest to the surface.
The water in them is replenished by underground rivers such as those that flow from the Atlas mountains to the west. And in Egypt, new underground rivers have been detected by satellite sensors like those used to hunt for water on Mars.
To get truly majestic underground lakes and rivers takes some specific geology. Because the Earth is basically a solid body, you need some soluble rock. One candidate is salt, sodium chloride. It exists in the Earth's crust in domes, needles and other shapes, and is extracted for human use by being dissolved in water and pumped out. Human salt mining has created spectacular caves in many parts of the world, notably the World Heritage Site at Wieliczka near Krakow in Poland. Caves exist in Spain and Israel that have been created in this way by nature, but they are a rarity.
Most of the world's natural caves were created by water, but using a different process. As we saw in Chapter 3, because the atmosphere contains carbon dioxide, which is slightly soluble in water, rain and river water do too. Let it loose on limestone, which is more or less pure calcium carbonate, and a slow' chemical reaction will produce calcium bicarbonate, which is soluble in water.
This means that anywhere in the world where you find limestone, you find caves. Because the caves are formed by water, they are essentially underground stream courses. Like streams on the surface, the water they contain is quick to rise when it rains hard. Many of the worst caving disasters happen when people are trapped by rising water.
However, many caves are now dry and completely safe. They get this way when a cave has been cut by water which has then dissolved an even lower passageway and left the upper one dry. To the true caver, a cave system has all the pleasures of a river system on the surface, but with the added attraction of being in three dimensions.
But even people who like to stay on the surface can learn a lot from the way water acts underground. Because it can move through rocks such as sandstone and limestone, springs and streams tend to appear where these rocks meet up with something less permeable such as shale. And a river vanishing into the ground is more or less a certain indicator of limestone, unless human engineers have been at work. So you can glance down a valley and draw your own mental map of its geology as you see water come and go. Along some valleys in Europe, you can map the underground world by looking at the villages – each marks a geological join which forced water to appear above ground and made human habitation possible.