THANH SON (VIETNAM), SEPT 18: The face of the missing pilot stares out from the black-and-white photo stuck to a bulletin board.Nearby, US soldiers and Vietnamese civilians dig methodically, under an anthropologist's supervision, seeking bones, teeth, or anything else that will confirm that the pilot was buried here after his jet crashed 33 years ago.It's grueling, expensive, and sometimes fruitless work. But the photo reminds the Americans that this was one of their own, a man with relatives who live with uncertainty.``When I'm sitting in a rocking chair with my grandkids, I can say I didn't serve in Vietnam, but I helped a lot of families,'' said Sgt George Hampton of Chicago, second-in-command of this dig.Overall, 2,081 Americans are still listed as missing in action 23 years after the Vietnam war ended. Many were known to be lost over water. Of those, 1,554 are in Vietnam, the rest are in Laos and Cambodia. Many veteran organisations in the United States were planning events today to honourthe missing. Defence secretary William Cohen was to preside over a Pentagon ceremony.All cases of ``last known alive'' have been investigated at least once, and US officials say there is no credible evidence that Americans still are being held. Vietnam says it handed over all prisoners of war in 1973.There are about four excavation operations each year in Vietnam, with several teams flown in from Hawaii to dig simultaneously.In between, the full-time US staff in Hanoi interviews witnesses, goes through documents released by the Vietnamese government and decides the best prospects for future searches.``Never before has there been such an intense, continuing effort to account for MiAs,'' said Lt Col Matt Martin, head of the US MiA office in Hanoi. The last digs, June 23-July 25, yielded seven sets of remains that were sent to the military's central identification laboratory in Hawaii for identification.The recovery work is getting harder. Most of the easier sites have already been examined. Urbansprawl covers some, others are under water. The hot, humid conditions speed decay.In one case, a team used 10,000 sandbags to divert a stream and rented an elephant to help with the heavy work.For 20 years, American concerns over Vietnam's cooperation in recovering the remains of MiAs came in the way of normalising relations between the two countries. The countries finally opened embassies in each other's Capital in 1995.Martin says the Vietnamese government has been cooperative, although occasionally there are problems dealing with local officials.Relations have improved dramatically since 1988, when the first US team was allowed to visit but its members were viewed as spies.The government has made it illegal to hold on to remains and has taken out newspaper ads encouraging people to cooperate, even though payment is limited to reasonable costs related to recovery.The current month-long search involves 16 sites - six primaries and ten alternatives - in ten provinces. As a team finishesone site, it moves on to another.As part of operation Rolling Thunder, several jets were providing cover for bombers hitting a north Vietnamese ammunition factory northwest of Hanoi when they came under attack from surface-to-air missiles.One F-4C was hit. The co-pilot ejected and was taken captive. After his release nearly eight years later, he said the pilot went down. Local villagers said they hurriedly buried the remains, fearing a follow-up air assault.The site already has been excavated twice. But memories fade, and the unmarked grave has been hard to find. The last dig found a tooth fragment, some 2.38-caliber rounds, a 1965 penny, part of a watchband, and a piece of a helmet shell, not enough to close the case.The digging is relatively shallow, using a trenching method that looks for aberrations in the soil indicating a burial site.While state-of-the-art technology like the ground-penetrating radar could be useful in some places, boulders and roots mean the work must be done byhand-picks, shovels and quarter-inch-gauge screens.When remains are found, there is a huge sense of relief. Often, the Vietnamese workers hold a Buddhist ceremony at the scene and sacrifice a water buffalo.``If this dig doesn't yield evidence, we'll get the witnesses back and go through it again,'' said Dr Brad Stern, the anthropologist on the twelve-person US team that supervises about 90 Vietnamese. But to come along 25-30 years later and ask them to try to find the site is difficult.''And at some point, if all the leads fail to pan out, there could be a decision to give up.``It's extremely sad, because in some cases, the family is not going to get their loved one back,'' Stern said. ``You give it your best shot.''