What have the thyroids of Australian sheep got to do with a nuclear explosion? Well, according to a study published in the journal Science & Global Security, the presence of iodine-131 within said sheep from the late 1970s is yet more evidence that the Vela Incident was indeed a secretive nuclear test.

This tale begins in 1963, with the signing of the Partial Test Ban Treaty (PTBT). Signed and ratified by most of the world (with the notable exception of China, among several others), it prohibited the test detonations of all nuclear weapons unless they were subterranean.

In order to make sure the Soviet Union complied with the PTBT, the US launched Project Vela, a monitoring program that involved satellites. On September 22, 1979, one of these satellites spotted a huge uptick in gamma and X-rays near the Prince Edward Islands – owned by South Africa – in the southern Indian Ocean, possibly consistent with a nuclear blast.

Although the Vela instruments could also pick up on natural electromagnetic radiation spikes, including those from deep space, the nuke hypothesis remains the most prominent.

Per Gizmodo, the key piece of evidence was the “double flash” spotted by the satellite, indicative of a nuclear blast. The intense heat of the blast briefly strips the air of electrons, meaning it is opaque for the shortest of moments before the shockwave pushes it out the way and the light can be seen again – hence, double flash.

It’s been largely thought by those in the know that the incident was a nuclear test by Israel, with apartheid-era South Africa playing a supporting role. The US, a staunch ally of Israel – which still denies responsibility – has long obfuscated on the issue. An otherwise cautious CIA report from 1980 notes that the yield of the atmospheric blast would have been about a fifth of that of the bomb dropped on Hiroshima.

Back in 2015, the Bulletin of the Atomic Scientists (BAS) explained that the mounting circumstantial and scientific evidence for such an event, although increasingly compelling, was yet to be conclusive.

A key part of the smorgasbord of evidence that’s been missing is the radioactive debris or fallout left behind by the blast. Such isotopes and products are well known, but rainy weather and the location of the suspected explosion mean that so far it hasn’t been detected.

That’s where this new study comes into play. It was authored by Dr Lars-Eric De Geer, an expert in tracking down sources of nuclear signatures and retired member of the Swedish Defence Research Agency, as well as physicist Dr Christopher Wright of the Australian Defence Force Academy.

A 2017 paper of theirs, published in the same journal, carefully rules out a meteoroid-satellite collision, whose impact potentially could have duped the Vela satellite.

This new study, using data first obtained by the University of Tennessee, notes that radioactive isotope iodine-131 was found in the thyroids of some Australian sheep. This indicates that they grazed in the path of “potential radioactive fallout plume” from around that time.

I-131 has a variety of sources, from medical facilities and power plants to – you’ve guessed it – nuclear explosions. Meteorological data from the time, assessed by the US National Oceanic and Atmospheric Administration and the Naval Research Laboratory, suggests that fallout from the blast would have indeed drifted over southern Australia.

That’s not all: various militaries including the US and the UK have listening stations on a number of remote islands in the region, including Ascension Island. Recently declassified data reveals acoustics that evidence a nuclear explosion near the Prince Edward Islands. A second signal picked up by the Navy’s underwater Sound Surveillance System near Newfoundland corroborates this location.

Altogether, the BAS’s assessment of the paper – which doesn’t seek to assign blame for the incident – notes that it “removes virtually all doubt that the ‘flash’ was a nuclear explosion.”