When workers at the Hanford Nuclear Reservation in southeastern Washington state suddenly found a hole in the ground on Tuesday morning, there was cause for concern.

Nowhere in the United States is there more nuclear waste and radioactive contamination than at Hanford, which has been the focus of a massive, complex cleanup effort by the U.S. Department of Energy since 1989. The site was used for decades to produce weapons-grade nuclear fuel, beginning with the Manhattan Project and the Second World War.

It just so happened that this hole — a "subsidence of soil" according to the Energy Department — had appeared above a concrete tunnel with radioactive equipment stored inside. The concern was that if the roof of the tunnel collapsed it could release long-trapped contaminated material into the air.

While a portion of the tunnel roof did collapse, officials said "there is still no indication of a release of contamination from the hole." But that's not to say the site — which was never designed to hold this much waste for as long as it has — doesn't pose other environmental risks.

200 million litres of radioactive waste housed in underground tanks 0:47

Hanford? Never heard of it. How did it get this bad?

The founding of the Hanford site dates back to 1943. It was created by the U.S. government to produce huge amounts of plutonium, which is used in the creation of nuclear weapons. Notably, plutonium produced at Hanford was used in the bomb U.S. military forces dropped on Nagasaki, Japan.

At its peak, the site consisted of nine nuclear reactors and four reprocessing plants, designed to turn uranium into plutonium. It's estimated that nearly two-thirds of the plutonium used by the U.S. government during Hanford's lifespan — and a staggering amount of related waste — was produced at this one site.

One of Hanford's reprocessing plants, the Plutonium Uranium Extraction plant, or PUREX, was built from 1953 to 1955, and is believed to have processed anywhere from 70 to 75 per cent of the plutonium at the site. The tunnel that collapsed on Tuesday was part of the PUREX plant.

What's the tunnel for?

Inevitably, the activities at Hanford resulted in the radioactive contamination of various pieces of equipment — notably, large machines including trains and railroad cars used to transport fuel to the PUREX plant for processing. Like the environmental waste Hanford produced, this equipment needed to be stored long term in relative safety, too. So workers began putting the contaminated equipment in a pair of concrete tunnels in the 1950s, which are buried in the soil more than two metres deep.

A photo released on the Hanford Emergency Information website shows the hole in the roof of a tunnel that is used to store radioactive equipment such as railcars and trains. (Hanford Emergency Information)

That mostly worked until Tuesday, when a hole roughly six metres in diameter opened up above a joint between the two tunnels. It's not clear how, exactly, the rift occurred, but one theory is that vibrations from nearby roadwork may have contributed to the tunnel's collapse.

How will they clean up this mess?

The PUREX facility — which includes the equipment in the two tunnels — was already slated to be decontaminated, demolished and buried. But for the tunnels in particular, the government has also been considering another option: entomb them in place with concrete, a process known as grouting.

"The cementation grout process is one that's becoming more and more favourably looked upon, as it's a cheaper, easier to deploy method," explained John Luxat, a McMaster University engineering professor and nuclear safety specialist.

How those plans will proceed now that the tunnel has been breached remains unclear. But the reality is that Hanford, and the PUREX facility in particular, have long had bigger concerns.

Worse than a decades-old tunnel leaking radioactive dust into the air? Go on.

How about decades-old tanks leaking highly radioactive liquid into the ground below?

The PUREX plant "generated quite a significant amount of waste material which was stored onsite, primarily in tanks," Luxat said. "And that proved to be one of the major issues when they came to decommission and clean the site. The tanks were decades old and there were issues around leakage."

The 177 large underground tanks hold some 200 million litres of radioactive waste. The tanks are decades old, with some long past the end of their operational life. The most recent leak was detected last year, but at least 3.8 million litres is believed to have leaked into the ground in the past, which may put things into perspective.

This week's tunnel collapse is just one of many radioactive headaches.