The water in the aquifer dubbed Ohangwena II, which lies under the boundary between Angola and Namibia, is up to 10,000 years old but safer to drink than many modern sources, scientists say.
On the Namibian side of the border the aquifer covers an area of about 43 miles by 25 miles.
"The amount of stored water would equal the current supply of this area in northern Namibia for 400 years, which has about 40 percent of the nation's population," said Martin Quinger from the German federal institute for geoscience and natural resources, which has been helping the Namibia government in its search for sustainable water supplies.
The 800,000 people who live in the area currently obtain their drinking water from a 40-year-old canal that carries the scarce resource across the border from Angola, the BBC reported.
The natural pressure in the aquifer would make it easy and inexpensive to extract, Quinger said, but a smaller salty aquifer that sits above the new discovery means careful drilling would be required to avoid contamination of the freshwater aquifer.
"If people don't comply with our technical recommendations they might create a hydraulic shortcut between the two aquifers which might lead to the salty water from the upper one contaminating the deep one or vice versa," he said.
"What we are aiming at is a sustainable water supply so we only extract the amount of water that is being recharged."
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