For the first time ever, scientists have mapped out the underground reservoirs of water throughout Africa and found that the northern part of the continent has the most groundwater stored.

Contrary to the common notions, beneath Africa's vast desert landscapes there exists about 0.66 million km3 of groundwater, a hundred times more than can be found on the surface and twenty times more than in Africa's lakes.

Researchers from the British Geological Survey (BGS) and University College-London mapped out the groundwater reservoirs, also known as aquifers, by cross-referencing a geological base map of the African continent with previously published maps of the African continent's aquifers, in addition to conducting studies of 283 African aquifers.

The results were the first quantitative groundwater maps for Africa.

As the climate became drier, transforming the Sahara into a desert, water was trapped underneath the surface in these aquifers more than 5,000 years ago.

In Africa, more than 300 million people do not have access to safe drinking water. The discovery of these reservoirs offer hope for the continent, provided that the water resources are not exhausted by recurrent droughts.

Much lower storage aquifers are present across much of sub-Saharan Africa. 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, Helen Bonsor, co-author of the BGS report, told the BBC.

However, these underground acquifers have not been replenished for an estimated 5,000 years due to the lack of rain, meaning that the stored groundwater is of limited supply. This fact has raised debate over how to best extract the water for drinking purposes.

Generally, groundwater is accessed by drilling boreholes, deep and narrow holes.

However, caution must be exercised given the limited supply. Wide developments of boreholes could actually deplete the aquifers.

It is not as simple as drilling big bore-holes and seeing rice fields spring up everywhere. In some places it could be economically and technically feasible to use groundwater to reduce crop loss, but I would question whether that is true everywhere. It will need detailed evaluation, Dr Stephen Foster, a London-based senior adviser for aid group Global Water Partnership told Reuters.

In addition the yield of boreholes, as in how much water is delivered from pumping, might not be enough for large-scale irrigation.

The uneven groundwater distribution is another cause for debate and potentially violent conflict between African countries as they become more desperate for water.

The discovery of groundwater under the notoriously dry continent Africa could dramatically affect its population, but practical access and extraction of the water could further delay the thirst relief.