Today, the 29th of August, 2018, marks the ninth annual International Day Against Nuclear Tests. It's a day of remembrance that was established by the United Nations in 2009 to raise awareness of "the effects of nuclear weapon test explosions or any other nuclear explosions, and the need for their cessation as one of the means of achieving the goal of a nuclear-weapon-free world".

Most people are familiar with the two bombs that were dropped on Hiroshima and Nagasaki in 1945, but little is known about the 2,000 nuclear bombs that have been detonated all across the planet in the years that followed. Not to destroy a specific military target, but just to measure their explosive power.

These atmospheric and underwater tests were often carried out in the presence of so-called "atomic veterans" – soldiers stationed near the explosion to test their force's readiness in response to a nuclear attack.

Estimates vary, but in the US alone, around 40,000 soldiers – mainly men – are believed to have watched a nuclear bomb explosion up close. We know this because the American military are relatively transparent about their nuclear programmes. A lesser known fact, though, is that European nuclear powers, like France and the UK, have also sent soldiers to far-off lands as guinea pigs.

After the US and Russia, the UK became the third nation to develop nuclear weapons – conducting hundreds of tests during the 1950s and 1960s. Many of these controlled explosions took place in Maralinga, a remote area in Australia mainly inhabited by a local indigenous people. Meanwhile, nuclear explosions involving larger hydrogen bombs were detonated in the Indian Ocean, off the coast of Australia's Christmas Island. On the 28th of April, 1958, the British military exploded its most powerful nuclear bomb, Grapple Y – which was around 200 times more powerful than the one dropped on Hiroshima.

The vast majority of the 20,000 British soldiers who were present when these top secret tests were done were not told beforehand that they would be participating in nuclear tests. On returning home, the soldiers were on strict orders not to discuss the exercises with anyone, keeping the tests hidden from the general public. In the decades since, many of these veterans have suffered from cancer and other illnesses that can be linked to exposure to extreme radiation.

Motherboard's new film, Atomic Soldiers: What Does a Nuclear Bomb Explosion Feel Like? was made to ensure these veterans' memories of experiencing a nuclear explosion won't be lost forever.

As part of the documentary, we spoke to atomic veterans at the British Nuclear Test Veterans Association (BNTVA) conference in Weston Super-Mare, near Bristol, to better understand how the tests impacted the ex-soldiers' physical and mental health.

Very few people today truly understand the full effects of nuclear weapons. In most countries, nuclear testing ceased in 1963, when the largest nuclear powers signed a limited nuclear agreement. Then, in 1996, the Comprehensive Nuclear-Test-Ban Treaty (CTBT) was signed, banning all nuclear explosions anywhere in the world. For a short while in the 1990s, the world seemed to be heading towards a nuclear-free future.

However, not all countries signed the 1996 treaty. And after a strong focus on nuclear disarmament decreased the number of nuclear bombs from 70,000 in the late 1980s to around 10,000 warheads today, we are again creeping towards more nuclear activity, especially as North Korea, China, Pakistan and India work towards developing and expanding their nuclear arsenals. Meanwhile, political leaders continue to normalise the use of these weapons by publicly threatening each other with complete annihilation.