NEAR WELWYN GARDEN CITY, England — An old toilet and a rusty kitchen sink sit amid a massive heap of junk covering the floor of an ancient beech forest.

Mattresses, old fridges, asbestos, office chairs and even a pink children's slide are mixed in with household and construction waste — with a mansion on the historic Brocket Hall Estate just a short walk away.

An estimated 485 tons of trash were dumped in the greenery here over a five-day period last summer.

Dating to 1760, Brocket Hall has been home to two British prime ministers and describes itself as a "favorite country retreat of royalty." Susannah Ireland / for NBC News

Michael Longshaw, the estate’s managing director, says "an enormous, almost like a military operation" was required to get so much garbage there in such a short time. He is now bracing for a cleanup bill expected to reach £200,000 ($258,000).

There is big money to be made in illegal dumping, and it can be made fast. And England's Environment Agency says "networks of career criminals" are increasingly getting involved in illegal dumping.

"They are not specialized waste crime criminals," North Yorkshire Police and Crime Commissioner Julia Mulligan told NBC News. "They are largely organized criminals looking at ways in which they can make and launder money. This is another string to their bow, if you like.”

Bluebells grow near piles of trash dumped near the Brocket Hall Estate, which is located around 22 miles north of central London. Susannah Ireland / for NBC News

The Environment Agency says criminals resort to intimidation and even violence to gain access to potential dumping sites.

Many victims are reluctant to go on the record about how they were targeted due to fear of reprisals.

But some like Stuart Roberts, vice president of the National Farmers' Union for England and Wales, recounted the steps being taken to protect their land from the gangs.

Trenches have been built around some properties, while others resort to installing reinforced gates and surveillance cameras or blocking access routes with slabs of concrete.

“We have to take defending ourselves into our own hands,” Roberts said.

At Brocket Hall, bluebells have bloomed around the garbage. A moat will be dug around the area once the waste is cleared.

“It’s almost like going back to the Middle Ages,” Longshaw added.

Michael Longshaw is the managing director of the Brocket Hall Estate. Susannah Ireland / for NBC News

Because of the huge profits and light penalties, the head of the Environment Agency has called waste crime the "new narcotics." Sir James Bevan compared the current situation to how Britain once struggled to tackle that illegal trade.

“It feels to me like drugs felt in the 1980s," Bevan added. "The system hadn’t quite woken up to the enormity of what was going on and was racing to catch up.”

While those convicted can go to jail, fines are often low in comparison to large profits that can be reaped. Few cases result in serious sentences, making the crime extremely lucrative and relatively low-risk.

The money-making model criminals use is relatively simple.

Legitimate waste carriers charge their customers a fee of several hundred pounds for the removal of each ton of waste. Almost half of that fee is paid to a licensed transfer station that then sorts and disposes of the waste.

But criminals are offering to take waste at lower prices and then dump it at farms, industrial sites or on estates, such as at Brocket Hall.

Brocket Hall Estate employee Andrew Scott surveys the junk he discovered last year. "It's disgusting," he said. "I still have no idea if it happens again, how we are going to stop it. It’s a nightmare." Susannah Ireland / for NBC News

They pocket the entire fee and leave landowners with hefty bills to remove the junk.

Based on anecdotal information about the rates typically charged by criminal operators, it's estimated the culprits who dumped hundreds of tons of waste at the Brocket Hall Estate made at least £20,000 (nearly $26,000) from that location alone.

"And we are just one of the sites selected on that particular day," Longshaw said.

The Environment Agency estimates illegal waste and the criminals behind such crimes divert as much as £1 billion ($1.3 billion) every year from legitimate businesses and Britain's Treasury.

An independent review into serious and organized waste crime commissioned by the U.K. government last year found that some criminals go further. The U.K.’s environment minister has warned that some involved in waste crime use it as a cover for human trafficking, drug running and money laundering.

Farmers are often the easiest prey. When security measures such as moats fail, Roberts says there is not much that can be done to protect themselves and their land.

“You don’t go near those people,” he said. “You basically just report it to the police and look the other way because you can’t confront them."

Roberts said even if someone is caught and prosecuted, the fines they face are often laughable. “So actually, for them it’s almost a commercial cost,” he said.

Lizzie Noel, who chaired the government's independent review into waste crime, said tracing criminals back to the waste they dump can be tricky for Environment Agency investigators.

"Part of the problem is that it’s not the police’s principle responsibility — the responsibility for investigating is spread across multiple agencies," Mulligan, who assisted Noel with the review, added. "That complexity causes problems."

A pink children's slide is among the piles of waste left on the Brocket Hall Estate. Susannah Ireland / for NBC News

The review also found it can cost anywhere from £10,000 to £500,000 ($13,000 to $660,000) to clear a single dump of waste.

And the criminals often return with more junk once a site has been cleared, repeating the cycle.

At Brocket Hall Estate, the area where the waste was dumped has now been fenced off and signs on trees advise that the site is being monitored.

Longshaw wants the culprits to be held responsible for the costs incurred to return the land to idyllic English countryside, but no one has been arrested in the case.

“I think there should be a mechanism put in place whereby the perpetrators of the crime pay for the cleanup. Simple as that,” he said.