The roadside bomb has evolved steadily in step with the defences deployed against it to become the most dangerous threat facing US and British troops in Iraq.
The roadside bomb is the biggest killer of US and British troops
An armour-piercing version of the bomb - blamed for the deaths of eight British soldiers this year - marks the latest advance in the insurgents' arsenal.
The UK has accused Iran of supplying the new weapon to militants in southern Iraq, via the Lebanese Hezbollah militia group, although Tehran has denied this.
According to defence sources, basic armour-piercing weapons are easy to manufacture, drawing on principles discovered more than a century ago and in use since World War Two.
'Hot knife in butter'
The roadside bomb - known by its military acronym, IED, for improvised explosive device - has come a long way since the early days of the US-led invasion in the spring of 2003.
Back then, according to defence sources, the typical IED was a crude affair, little more than an artillery shell or mine booby-trapped to ensnare passing soldiers.
The blast would scatter shrapnel indiscriminately, injuring "soft" targets such as civilians.
Evidence suggests insurgents are using more complex bombs
Against the heavily-armoured vehicles favoured by the foreign armies in Iraq, it proved less effective. Enter the "shaped charge", an IED adapted to focus the force of the blast.
"It'll go through the heaviest armour like a hot knife through butter," John Pike, director of US defence policy group Globalsecurity.org, told the BBC News website.
Whereas the standard bomb is little more than an explosive inside a metal cylinder, possibly lined with metal ball bearings, the explosive in a shaped charge has a cone-shaped hollow cut into it.
The interior of the hollow is lined with a metal that has a low melting-point, usually copper. With the broad, open end of the cone facing the target, the explosive is detonated, compressing the copper into a jet of speeding metal.
"The single, self-forging fragment hits the target at a high velocity," Mr Pike says.
Several existing munitions, including anti-tank bazookas and rocket-propelled grenade launchers, operate on a similar principle, known as the Munroe effect after Charles Munroe, the US navy scientist who discovered it in 1888.
Explosions that exploit the Munroe effect are commonly used in the civilian sphere to demolish buildings and cut steel girders.
Of course, says Mr Pike, not all armour-piercing blasts in Iraq are the work of shaped charges.
The country is awash in ammunition, he says, so a huge amount of explosives can be easily packed into a bomb and prove just as devastating.
Southern Iraq has seen a rise in roadside bombings
An explosion in August that killed 14 marines and their translator, up-ending their 27-ton amphibious assault vehicle, was initially thought to have used a shaped charge.
But US military investigators are said to have found the sheer mass of explosive used - rather than its design - destroyed the vehicle.
In January, Brig Gen David Rodriguez warned that insurgents were puncturing US defences by packing ever larger amounts of explosives into their bombs.
He said boosting the armour on US vehicles was not enough to cut losses - the army needed to adapt its tactics and acquire better intelligence.
According to Charles Heyman, a defence analyst with the UK-based Jane's Information Group, convoys and patrols are the Achilles heel of the modern army.
The insurgents, he says, have learnt that military vehicles have the heaviest armour along the under-body, in areas where they are traditionally most likely to be struck.
In response, bombs in Iraq have been fastened to telegraph poles and the underside of bridges, directing the blast at the relatively exposed top and sides of army vehicles.
"Concealing something by the roadside is just too easy," he told the BBC News website.
Abandoned cars, piles of roadside rubbish, ditches and craters left by previous blasts are among the various sites where explosives have reportedly been concealed.
With armour offering limited protection from an explosion, US and British forces have tried a variety of tactics to disrupt the trigger mechanism.
Modern armies are at their most exposed on the roads, experts say
Large convoys are equipped with expensive radio-jamming devices that paralyse the remote signal used to set off an IED.
According to Mr Heyman, the success of the jammers has forced most Iraqi insurgents to abandon electronic triggers.
Many roadside bombs, he says, now rely on "command detonation" - a physical link between the bomber and the bomb that can be a concealed cable, or equally, just a long piece of string.
Trip-wires have also been used, stretched across a street sometimes just seconds before a military vehicle passes by.
Recent attacks on British forces in southern Iraq have used an electronic version of the trip-wire that is immune to radio jamming devices - the infrared beam, of the kind commonly seen in burglar alarms.
The beam triggers the blast when it is broken by a passing vehicle and is harder to disrupt using radio jamming technology.
With militants racing to perfect the roadside bomb, analysts say the military's best defence is probably the simplest one - a better network of observers and informers.