IT COULD BE a giant archaeological dig. Bulldozers tear at the jungle to reclaim the history of the second world war and its dark finale: the atomic bombing of Hiroshima and Nagasaki 80 years ago this month. The work on Tinian, a speck in the Pacific Ocean, has exposed the four runways of North Field. Glass protects the cement pits where Little Boy and Fat Man, the first and only atom bombs used in war, were loaded onto American B-29s. For a time Tinian was the largest air base in the world, but it was soon mostly abandoned.