Scientists have for the first time mapped the distribution and scale of the vast amounts of water sitting under the Sahara and other parts of Africa.
They say the waters are contained in ancient aquifers that were last filled 5000 years ago.
Researchers say these groundwater sources could provide enough for the whole of Africa, where more than 300 million people are said not to have access to safe drinking water.
Total volume of water in aquifers underground is 100 times the amount found on the surface, according to the study published in a journal, Environmental Research Letters.
Researchers from the British Geological Survey (BGS) and University College London (UCL) say that large scale drilling might not be the best way of increasing water supplies on a continent where at present only 5% of arable land is irrigated.
"Where there's greatest ground water storage is in northern Africa, in the large sedimentary basins, in Libya, Algeria and Chad," says Helen Bonsor from the BGS - one of the authors of the paper - told the BBC.
"Storage in those basins is equivalent to 75m thickness of water across that area - it's a huge amount."
The scientists collated their information from existing hydro-geological maps from national governments as well as 283 aquifer studies.
Reservoirs for drought-prone nations
New maps indicate that many countries currently designated as "water scarce" have substantial groundwater reserves.
Dr Alan MacDonald of the BGS, lead author of the study, told the BBC: "High-yielding boreholes should not be developed without a thorough understanding of the local groundwater conditions.
"Appropriately sited and developed boreholes for low yielding rural water supply and hand pumps are likely to be successful."
Large-scale borehole developments could rapidly deplete the resource, and Dr Bonsor says sometimes the slower means of extraction can be more efficient.
"Much lower storage aquifers are present across much of sub-Saharan Africa," she explained.
"However, our work shows that with careful exploring and construction, there is sufficient groundwater under Africa to support low yielding water supplies for drinking and community irrigation."
"At present extraction rates for drinking and small scale irrigation for agriculture groundwater will provide and will continue to provide a buffer to climate variability."