In the darkest hours of tropical nights, professor Zenobia Jacobs trekked into the scrub of Kakadu National Park in search of the tiniest of time-capsules.

Her job was to collect grains of sand from an archaeological dig within an ancient Aboriginal rock shelter known as Madjedbebe.

She collected samples every five centimetres as the dig progressed right down to 2.5 metres.

She could not expose a single grain to sunlight.

A grain of mineral is, in effect, a battery. It discharges when exposed to sunlight, but as it lies beneath the ground, deprived of light, it charges up from the radiation that exists in all soils and rocks. 

The longer it lies in the dark Earth, the greater the charge. It keeps perfect track of time.

Professor Jacobs, once she had spirited her samples to her laboratory at the University of Wollongong, employed a technique known as "single-grain optically stimulated luminescence". 

She stimulated the charge by exposing each grain to the light of a laser, forcing it to release its stored electrons.

Professor Jacobs measured, one by one, the tiny charge from 25,800 separate grains of sand.

By dividing the charge by the radiation known to have existed in the ground from which the sample was taken, an accurate measurement emerged of the time elapsed since the grain of sand had last been exposed to sunlight.

Professor Jacobs' work established that Madjedbebe was vastly older than imagined: 65,000 years.

To be doubly sure, she submitted four samples from different levels - which her work had measured at 18,000, 35,000, 50,000 and 65,000 years - for independent testing. Dr Lee Arnold, a dating expert from Adelaide University, was not told the origin of the samples or what work they were associated with before he tested them.

"And they came back as a perfect match to my measurements," beams Dr Jacobs.

Tony Wright travelled as a guest of the Gundjeihmi Aboriginal Corporation.