The Aral Sea  is an endorheic inland sea in Central Asia; it lies between Kazakhstan in the north and Karakalpakstan, an autonomous region of Uzbekistan, in the south. Since the 1960s the Aral Sea has been shrinking, as the rivers that feed it (the Amu Darya and the Syr Darya) were diverted by the Soviet Union for irrigation. The Aral Sea is heavily polluted, largely as the result of weapons testing, industrial projects, and fertilizer runoff before and after the breakup of the Soviet Union.

The major ecological problem is that diversion of the Amu Darya and Syr Darya rivers for irrigation has shrunk the Aral Sea dramatically; the Aral Sea has been drying up for about 40 years. This has brought about a number of ecological and economic problems for the sea and the surrounding area.

The Soviet Union decided in 1918 that the two rivers that fed the Aral Sea, the Amu Darya in the south and the Syr Darya in the northeast, would be diverted to try to irrigate the desert, in order to grow rice, melons, cereal, and also, cotton; this was part of the Soviet plan for cotton, or "white gold", to become a major export. (This did eventually end up becoming the case, and today Uzbekistan is one of the world's biggest exporters of cotton.)

The irrigation canals began to be built on a large scale in the 1930s. Many of the irrigation canals were poorly built, letting water leak out or evaporate; from the Kara Kum canal, the largest in Central Asia, perhaps 30–70% of the water went to waste. Today only 12% of Uzbekistan's irrigation canal length is waterproofed.
By 1960, somewhere between 20 and 50 cubic kilometers of water were going each year to the land instead of the sea. Thus, most of the sea's water supply had now been diverted, and in the 1960s the Aral Sea began to shrink. From 1961 to 1970, the Aral's sea level fell at an average of 20 cm a year; in the 1970s, the average rate nearly tripled to 50–60 cm per year, and by the 1980s it continued to drop, now with a mean of 80–90 cm each year. Even seeing this, the rate of water usage for irrigation continued to increase: the amount of water taken from the rivers doubled between 1960 and 1980; cotton production nearly doubled in the same period.

The disappearance of the lake was no surprise to the Soviets; they expected it to happen long before. The Soviet Union apparently considered the Aral to be "nature's error", and a Soviet engineer said in 1968 that "it is obvious to everyone that the evaporation of the Aral Sea is inevitable".

The sea's surface area has now shrunk by approximately 60% and its volume by almost 80%. In 1960, the Aral Sea was the world's fourth-largest lake, with an area of approximately 68,000 square km and a volume of 1100 cubic km; by 1998, it had dropped to 28,687 square km, and eighth-largest. Over the same time period its salinity has increased from about 10 g/l to about 45 g/l. As of 2004, the Aral Sea's surface area is 17,160 square km, 25% of its original size, and still contracting.

In 1987, the continuing shrinkage split the lake into two separate bodies of water, the North Aral Sea and the South Aral Sea; an artificial channel was dug to connect them, but that connection was gone by 1999 as the two seas continued to shrink.
Work is being done to preserve the North Aral Sea, including the construction of dams to ensure that fresh water continues to flow into it. In October 2003, the Kazakh government announced a plan to build a concrete dam (Dike Kokaral) separating the two halves of the Aral Sea, in order to raise the water level of the North Aral Sea and reduce its salinity. For financial reasons, the South Aral Sea has been abandoned to its fate. As it dries, it is leaving behind vast salt plains, producing dust storms, and making regional winters colder and summers hotter. Attempts to mitigate these effects include planting vegetation in the newly exposed seabed. As of 2006, some recovery of sea level has been recorded."The dam has caused the small Aral's sea level to rise swiftly to 125 feet, from a low of less than 98 feet, with 138 considered the level of viability."

As of summer 2003, the South Aral Sea is vanishing faster than predicted. In the deepest parts of the sea, the bottom waters are saltier than the top, and not mixing. Thus, only the top of the sea is heated in the summer, and it evaporates faster than would otherwise be expected. Based on the recent data, the western part of the South Aral Sea is expected to be gone within 15 years; the eastern part could last indefinitely.

The ecosystem of the Aral Sea and the river deltas feeding into it has been nearly destroyed, not least because of the much higher salinity. The land around the Aral Sea is also heavily polluted, and the people living in the area are suffering from a lack of fresh water, as well as from a number of other health problems—the receding sea has left huge plains covered with salt and toxic chemicals, which are picked up, carried away by the wind as toxic dust, and spread to the surrounding area; the population around the Aral Sea now shows high rates of certain forms of cancer and lung diseases, as well as other diseases. Crops in the region are also destroyed by salt being deposited onto the land. The town of Moynaq in Uzbekistan had at one point a thriving harbor and fishing industry employing approximately 60,000 people; now the town lies miles from the shore. Fishing boats lie scattered on dry land that was once covered by water; many of them have been there for 20 years. The only significant fishing company left in the area has its fish shipped from the Baltic Sea, thousands of kilometers away.

Many different solutions to the different problems have been suggested, ranging in feasibility and cost. These include:

In January 1994, the countries of Kazakhstan, Uzbekistan, Turkmenistan, Tajikistan and Kyrgyzstan signed a deal to pledge 1% of their budgets to helping the sea recover.