CBS News correspondent Terry McCarthy was with one of those teams in the southern town of Safar Bazaar in Helmand province - and saw firsthand the harrowing danger it faces day after day.
In the chaotic moments after a bomb blast, Marines run with stretchers to help their fallen comrades. An explosion went off behind a building. Moments later, a radio call comes in saying that two Marines are down.
Two men are carried away, a third, less seriously wounded, manages to walk out. They are two bomb disposal experts - known as an Explosive Ordnance Disposal (EOD) team - and a combat engineer.
Captain Nathan Opie rushes to the scene and gets the bad news from one of his men.
An emotional Capt. Opie described the scene: "He said that they got Greer and they got him pretty bad. At that point he brought me over there to Greer and I saw the EOD techs as well and we just started getting them on stretchers and getting them out of there."
Corporal Daniel Greer, of the 4th Combat Engineer Battalion, suffered a serious brain injury which later claimed his life.
Sgt. Johnny Jones, an explosives disposal expert lost both his legs. Staff Sgt. Eric Chir, also a bomb expert, suffers serious shrapnel wounds.
In the week before this blast, we followed these three men closely as their unit occupied Safar Bazaar, which has been heavily mined by the Taliban.
As the initial wave enters the town, Sgt. Jones, wearing a helmet camera, sees the first bomb explode.
"Is everyone alright?" Sgt. Jones yells.
This time the Marines are lucky. No one is hurt. Jones and Chir examine the device that had gone off. They then find two more bombs that are ready to explode, and begin to slowly unearth them.
As they prepare to cut the wire to a detonator, a flash of humor breaks the tension.
"I love you man."
"Right now, I love you too."
Greer's job as an engineer is to open up safe routes for the other Marines to travel along. Often that means blasting holes in walls to avoid possible booby-traps
After four days inside the town, the Marines have already found 40 IEDs. Now, the work is slow and meticulous, and in areas that are laced with bombs the Marines walk along narrow paths which have been cleared. They're literally stepping in each other's footsteps.
When possible they use robots and explosives to try to detonate IEDs from a distance. Going in on foot is the last resort.
It is on day seven that Jones, Greer and Chir are blown up. They are medivacked out of the area. The Marines they leave behind are devastated.
How does one react to something like this?
"You kind of have to stay static on it, - I mean, later it will take effect," said Sgt. Matthew Jackson of the 1st EOD Company, 1st Marine Logistics Group. "You know we've had a relatively rough summer, but you just got to keep grinding."
Sgt. Jackson is another bomb disposal expert who worked closely with the team - he is asked to do the post-blast analysis.
"There was just something they didn't see, you know it's nobody's fault, you know, that's just how it happens," Jackson said.
The Marines will continue to press forward - that is what they do. They are gaining ground in Helmand, but the human costs are mounting. On the outside, they will hang tough. On the inside, the hurt is growing.
More of Terry McCarthy's "Thundering Third" Blogs:
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