To date the tunnel, the researchers measured samples of plants and wood contained within the plaster that lined the tunnel. Living plants contain a certain amount of radioactive carbon-14 that is replenished through the process of photosynthesis. Since this quantity starts to diminish once the plant dies, the ratio of carbon-14 to stable carbon-12 reveals how long the plant has been dead.

Frumkin and colleagues measured the relative masses of the carbon isotopes within the plaster using an accelerator mass spectrometer at Oxford University. They calculated that the material in the plaster dates from between about 800 to 510 BC.

The geologists then dated stalactite material - known to have formed after the tunnel was built – from the ceiling of the tunnel. They used uranium-thorium dating by thermal ionization mass spectrometery at the Open University in the UK. This method is based on the detection of both the parent (uranium-234) and daughter (thorium-230) products of decay by the emission of an alpha-particle. The decay of uranium-234 to thorium-230 is part of the much longer decay series that begins in uranium-238 and ends in lead-206

For this technique to work, researchers must know or calculate the initial ratio of thorium-230 to uranium-234 when the sample formed. With time, thorium-230 accumulates in the sample through radioactive decay. They calculate the sample’s age by the difference in the initial ratio of thorium-230 to uranium-234 and the one in the sample being dated.

Frumkin and colleagues found that the stalactites were more than 2300 years old. They therefore conclude that the tunnel could not have been built as late as the 2nd century BC, as some archaeologists believe.

“This is the first time a structure mentioned in the Bible has been radiometrically dated,” Frumkin told PhysicsWeb. “Our dating agrees well with that of the reign of King Hezekiah. Until now this presumption was based only on the biblical text and on the inscription found on the tunnel itself.”

The team now hopes that their technique will be used to date other historical and biblical structures.