The worlds inland seas and lakes have a unique allure. Little ones such as those in the English Lake District or the Alps have had the power to inspire artists and writers from William Wordsworth and Mary Shelley onwards. Think of the wild weekend on the stormy shores of Lake Geneva in 1816 when the teenage Shelley penned Frankenstein.

But inland seas have other uses besides the artistic. For a start, they hold almost all the Earths usable fresh water. Far more is contained in the ice caps of Greenland and Antarctica, but that water is locked up inaccessibly. To count as a lake, a body of water must be disconnected from the world ocean. Instead, lakes are fed by rain and snow falling directly on them or arriving via rivers. They lose water by evaporation and via outflowing rivers.

Making a lake is simplicity itself. Start with a depression in the land, and water will find its way there, rising until it reaches the lowest part of the rim, where a stream or river will drain out. This origin makes lakes far less saline than seas and oceans. But they are also more confined and vulnerable. On the up side, this means that many lakes have unique ecologies. Like remote islands, many are home to species whose ancestors found their way in millennia before and have since evolved in response to local selection pressure. The down side is that a lake cannot rid itself of pollution as readily as an ocean might. It is surprisingly easy for large lakes to be damaged by human activity.

The Sea of Aral: vanishing between 1989 (left) and 2003 (right). There are plans to seal off the dry areas permanently to help the rest refill.

A prime case is the Sea of Aral, now divided between Uzbekistan and Kazakhstan but wrecked by the Soviet Union. The Soviet eras worship of industrial production at all costs led to the rivers feeding the sea, the Amu-Dar’ja and the Syr-Dar’ja, being used to irrigate cotton fields. The diverted water mainly evaporated or was lost due to bad sealing of the irrigation channels. But as a result of this water abstraction the Sea of Aral turned from the fourth-largest lake on Earth to the eighth. It shrank from 68,000 square kilometres in 1960 to under 29,000 in 1998 and the process is not complete yet. It could be down to 21,000 by the year 2010 according to DLR, the German space agency, wrhose scientists have modelled its decline. Its salinity has increased tive-told, making its water less drinkable, while the total volume of water it contains has shrunk from over 1000 cubic kilometres to under 200.

In terms of economic activity, the decline of the Sea of Aral wrecked a fishing industry that used to produce over 40,000 tonnes of food a year. And as the lake has dried out, the salt it once contained has formed salt flats and blown across once-prosperous agricultural land. There are reports of increased respiratory disease and infant mortality among the surrounding human population to add to the depressing sight of big seagoing ships rusting in the sand, miles from any water.

The Soviet Union was also home to one of the worlds most remarkable bodies ot tresh water, Lake Baikal. Despite some pollution from wood-pulp processing, its remote Siberian location protected it from severe environmental damage.

Lake Baikal has been an independent mass of fresh water for 25 million years. In this time it has developed its own ecology with unique species of everything from fish to bacteria. It contains about a fifth of the worlds available fresh water, and runs down to over 1600m deep. Its watershed, the area that drains into it, covers 540,000 square kilometres. Most of it is rocky so the water entering the lake is very pure, which accounts for the lakes exceptionally clear water.

In recent years, some answers have been emerging as to just why Lake Baikal has lasted so long in such a stable form. There had been a theory that the steep basin in which the lake sits is gradually subsiding. But a look at the bottom of the lake with a submersible shows that it is volcanically active and may be a site of slow tectonic spreading. The areas lively earthquake history supports this view, and the bottom of the lake exhibits volcanic vents like those seen at mid-ocean ridges. The lake may well be expanding at a rate of a few centimetres a year, a tar-off effect of the deformation that has raised the Himalayas to the west. At this rate, even a lkm change in its size would take millennia.

Lake Baikal is not the only place where the unquiet Earth has generated stable geography. In East Africa, something similar is happening in the Rift Valley. There Lake Tanganyika is the second-deepest lake on the planet, 1470m at its deepest. It has been isolated for about 10 million years. As a result it has about 600 unique species.

Nearby Lake Victoria can no longer make such a claim. Like other lakes in the area, it is home to unique species of fish from the cichlid family. There were 500 unique cichlid species in the lake, all descended from a common ancestor, until the 1950s when Nile perch were introduced to the lake in an attempt to improve fishing yields. These perch are able to grow up to 2m long, especially given a rich cichlid diet, and the species count is now nearer 250. Another Rift Valley lake. Lake Malawi, has fared better and is home to around 500 species. To see this in context, remember that Europe has about 60 species of freshwater fish in all its lakes and rivers put together. But at some point the spreading of the Rift Valley will mean that these lakes will meet up with other oceans and become salty, putting all these species in danger.

Tectonic processes do not have to produce vast expanses of deep water. Further north in the Rift Valley, in Kenya, the same forces have created shallow soda lakes. Like the bigger lakes to their south, these lakes are far inland. But about 70 million years ago, modest water bodies like these were probably present as the first symptom of the tectonic disturbance that marked the opening of the Atlantic.

However, big lakes can exist without volcanic turmoil, as evidenced by the Great Lakes, at the tectonically inert centre of North America. Put together, the five Great Lakes contain 23,000 cubic kilometres of water, about as much as Lake Baikal. The Great Lakes are at the centre of a continent, like Lake Baikal, in an area that exhibits the extreme weather variations that can be expected a long way from the ocean. In turn, they are big enough, with an area of 244,000 square kilometres, to make their own weather. In winter, winds that blow across them pick up water vapour that reappears as snow far across the surrounding area.

The Great Lakes lose water into the St Lawrence River and other outlets only very slowly. They drain about 1 percent of their total volume each year. In the biggest, Lake Superior, the water is replaced only once in 191 years, but is very pure because it is comparatively tar from cities and industry. In Lake Erie, the smallest of the five, the water is replaced much more quickly once every 2.6 years – but it is tar more polluted by people, factories and agriculture.

Nor do lakes have to be on a spectacular scale to be rewarding. In the British Isles, the Alps, Scandinavia and other parts of Europe, many exist in mountain areas shaped by recent ice ages. An example is the Lake District in England. Its lakes are mainly in valleys carved by ice, which gives some of them a confined and sometimes gloomy appearance.

Here and in other mountain areas of the world, some of the most pleasing valleys have attracted the attention of water engineers who have improved on nature by constructing dams in areas where any other form of industrial development would be regarded as utterly inappropriate. Near major population centres (Manchester in the case of the Lake District, San Francisco for the Sierra Nevada of California), most of these dams and the reservoirs they retain are there to provide drinking water.

In more remote areas, as in the Alps and Scandinavia, artificial lakes have been created, and existing ones expanded, to provide hydro-electricity. Lakes do not have to be big to be important. Because the Earths living systems are driven by water, even small ponds are vital to the ecology of both wet and dry areas. But big lakes have a very specific role in the Earths water cycles. They aggregate water from many sources – Lake Baikal is said to have 544 tributaries. But they distribute it in concentrated form. In almost all cases they produce a single river, often a very large one such as the Nile, whose sources are in the great lakes of East Africa.