The four Pentagon scientists gathered at a secret base about an hour's drive from Washington, DC. The three younger scientists wore camouflage jackets and dark, wraparound sunglasses. The fourth, their leader, was a 61-year-old man named Bob Best who has thick eyeglasses and thinning hair. He was about to build a bomb.

It wouldn't be his first. Over the past five years, Best has made and blown up more than 9072 kilograms of homemade explosives using formulas cribbed from insurgents in Afghanistan and Iraq. But, this bomb would be different. This bomb, he hoped, wouldn't explode.

The bomb would be made with a new variant of ammonium nitrate fertiliser that had been touted as non-detonable. For Pentagon scientists, such as Best, the search for a fertiliser that doesn't explode has been a Holy Grail-like quest. Ammonium nitrate, which packs a fearsome punch, is used in more than 60 per cent of the Taliban's bombs. It's also essential to farming; without it, thousands of Afghans and Pakistanis would starve.

This northern hemisphere spring, an engineer at Sandia National Laboratories in New Mexico, announced he had found a special additive that blunted the fertiliser's blast without damaging crop yields. Sandia trumpeted the breakthrough in a news release. "Fertiliser that fizzles in a homemade bomb could save lives around the world," the federally funded research laboratory promised.

But Sandia can't test bombs, which is a tightly regulated activity. Best would give the formula its first official try. He whipped up a batch of the Sandia fertiliser and took it to his test facility, one of the more secretly guarded places in Washington. The range is at the end of a narrow, two-lane road and is surrounded by woods, family farms and a church advertising a "weekend Shad bake". There's no sign at the base's entrance, just a big metal fence, lots of barbed wire and several video cameras.

Past the fence, the facility resembles a post-apocalyptic junkyard. The charred carcasses of old cars, blown up in previous tests, litter the grounds. A rusted fighter jet that is missing its landing gear lies in a field, partially obscured by weeds. The occasional bald eagle circles overhead.

"We can operate out here and not attract a lot of attention," Best said.

"No one is going to ask any questions."

A light rain fell as Best and his team started to make two bombs in 19-litre plastic paint buckets. The first bomb was made from traditional ammonium nitrate fertiliser - the kind that's used by insurgents every day in Afghanistan. The other bomb contained the Sandia fertiliser, which includes an iron sulfate additive that is supposed to split the ammonium nitrate into two nonexplosive compounds: iron nitrate and ammonium sulfate.

The iron sulfate gave the Sandia fertiliser a light greenish tint. One of Best's scientists worked silently, pouring fine aluminum powder, which fuels the blast, into the two plastic buckets of fertiliser.

"Dump the aluminum in there, stir it up and now you have a bomb," Best said.

"This is what our soldiers are up against right now."

Ten years ago, the bombs used in Iraq and Afghanistan were mostly old artillery shells that insurgents found in dumps and buried along regularly traveled roads. The military countered by adding layers of thick armour to their trucks. So began a decade-long game of cat and mouse; move and countermove.

Iraqi fighters, with help from Iran, built high-tech devices that could pierce the American armour. The Afghan insurgents turned to fertiliser bombs, which are cheap, easy to make and devilishly hard to detect. A typical bomb kills with razor-sharp shrapnel. A fertiliser bomb contains no metal. It kills with a wave of intense energy that passes through the thickest armour.

"The ears are the most sensitive," Best said, describing in more detail the damage the bombs could do, as two of his scientists mixed the ammonium nitrate and aluminum powder.

"It isn't the noise that blows the eardrum, but the overpressure from the bomb," he said. The lungs are typically the next to fail, followed by the brain and then the heart.

After about 10 minutes of work and a couple of additional steps, the bombs were done and ready to blow. Taliban insurgents usually make 18-kg fertiliser bombs in yellow, plastic cooking oil containers. Best's bombs were only 4.5kg.

"If we build them any bigger, we are more than likely to crack a window in someone's house," said the physicist on Best's team.

"The rich people out here have houses on the river with lots of windows."

The test range consists of a muddy field, two big dirt berms and two rusty shipping containers that serve as an office. Silver sensors shaped like microphones measure the pressure from the blast wave. High-speed cameras record the action at a rate of about 10,000 frames per second, fast enough on this day to track the movement of individual rain drops falling through the sky.

First up on the test range was the plastic bucket of untreated ammonium nitrate fertiliser, which one of Best's scientists set on a smooth steel plate. The plate - or, rather, any dent in it - would offer up a crude measure of the blast's power, even if the more sophisticated technology on the range failed.

Best and his scientists took shelter in one of the cramped shipping containers. Inside, plywood desks were crammed with knives, screwdrivers, hammers and a bag of stale Krispy Kreme donuts. Computers captured readings from the sensors.

"Ready," said the firing officer.

"On the count of 3, 2, 1. Fire."

The blast rippled through the air, strong enough that Best and his team could feel it vibrate in their sternums.

"Imagine if you had just stepped on that," Best said.

"We are 150 feet away (46 metres) and protected by two berms."

Data from the blast were recorded. Debris from the range was cleared. Now it was time to test the Sandia fertiliser mix.

One of Best's scientists set the plastic bucket on the range and retreated to the shipping container. If the Sandia mix worked as promised, it would be a huge breakthrough. Ammonium nitrate fertiliser has been used in bombs in dozens of countries in the past few years. In April, a Texas plant that was manufacturing the fertiliser exploded, killing 15 and wounding 200.

"Ready," said the firing officer.

"On the count of 3, 2, 1. Fire."

Another sternum-shaking boom.

Best and his team walked up a small hill to the range and examined the steel plate on which the bomb had been placed. No dent. Instead, there was a softball-sized hole.

"Not good," Best said.

He and his team returned to the shipping container to check the sensor's readings, which would tell them whether the Sandia mix had done any good at all.

With the wars drawing to a close and American combat casualties falling, the Defense Department is weighing whether to dismantle the division of the Pentagon where Best works and send its people back to the military services.

The division, known as the Joint Improvised Explosive Device Defeat Organisation, was established in 2006 when dozens of troops were being killed by insurgent bombs every month.

Even if it remains open, Best said that his job will likely be eliminated in January.

"I want to stay," he said.

"But I can't figure out how to stay."

One of Best's scientists checked the readings on the computer.

"It looked like it suppressed the overpressure from the blast by about 5 per cent," he said.

Best shook his head. He would need to call Sandia.

"You are still dead," he said.

"This was not a success. We still have a guy in a body bag."

- The Washington Post