A world without fresh water would be a world bereft of humans, and yet one in five people lacks regular access to this most basic of life-sustaining substances.
By 2025, fully a third of the planet's growing population could find itself scavenging for safe drinking water, the United Nations has warned ahead of World Water Day on Saturday.
More than two million people in developing countries -- the vast majority children -- die every year from diseases associated with unsanitary water.
There are a number of interlocking causes for this scourge.
Global economic growth, population pressures and the rise of mega-cities have all driven water use to record levels.
Mexico City, Jakarta and Bangkok, to name a few, have underground water sources -- some of them nonrenewable -- depleting at alarming rates.
In Beijing, home to 16 million, aquifers have fallen by more than a dozen metres (40 feet) in 30 years, forcing the government to earmark tens of billions of dollars for a scheme to ferry water from the Yangzte River in the south to the country's parched north.
Aggravating the shortages are pathogen and chemical pollution, which have transformed many primary sources of water in the developing world into toxic repositories of disease.
Desperation forces people to consume these contaminated waters.
"In the coming decades, water scarcity may be a watchword that prompts action ranging from wholesale population migration to war, unless new ways to supply clean water are found," comment a team of researchers in a review of water purification technology published Thursday in the British journal Nature.
But even as scientists and governments look for ways to satisfy a thirsty world, another threat looms on the horizon: global warming.
Rising sea levels are already forcing salt water into aquifers beneath megadeltas that are home to tens of millions, and changing weather patterns are set to intensify droughts in large swathes of Africa, southern Europe and Asia, according to UN's Intergovernmental Panel for Climate Change (IPCC).
Experts and policy makers point to three broad categories of initiatives to ease the shortage of clean, drinkable water, especially in the world's poorest regions: sanitation, purification, and water management.
"Poor sanitation combines with a lack of safe drinking water and inadequate hygiene to contribute to the terrible global death toll," UN Secretary-General Ban Ki-moon said earlier this month.
"Every 20 seconds, a child dies as a result of the abysmal sanitation conditions endured by some 2.6 billion people globally," he said in launching the International Year of Sanitation.
Less than half the households in major Asian cities are connected to sewers, which means that tonnes of raw sewage runs into rivers and oceans, according to the UN.
In Latin America and Africa that figure drops to 40 and 20 percent, respectively.
While governments attempt to improve sanitation infrastructure, scientists are developing new technology to purify the water available, said Mark Shannon, a professor at the University of Illinois and Director of the US government funded Center of Advanced Materials for the Purification of Water with Systems.
"Desalination with reverse osmosis is already the largest single growth area in terms of new water supplies," he told AFP in an interview.
New techniques of reverse osmosis use membranes with nanometer-size pores to filter out salt and other contaminants from water, and could for the first time pave the way for industrial-scale use.
Micro-filters are also used to decontaminate bodies of water increasingly laced with pesticides, arsenic, heavy metals, nitrates and pharmaceutical derivatives.
Current methods of decontamination, however, remain "challenging, expensive and unreliable," said Shannon, and will take years to perfect.
A third method of purification -- and the one most relevant to the poor nations -- is removing or killing bacteria, viruses and other pathogens through disinfection.
"Pathogens are still the biggest problem in the world today in terms of safe water," Shannon said.
With worldwide food production set to expand 50 percent by 2030, scientists are also developing genetically modified grain plants that consume less water and can withstand harsh conditions.
Researchers in the US, for example, have developed genetically engineered rice with a higher tolerance for drought, salt and low temperatures, the three main causes of crop failure.