It is explained how the artifacts in Maggie's yard were preserved by peat. The peat bogs of northern Europe are tens of thousands of years old; they are up to 15 or 20 feet deep layers of dead, waterlogged plant matter. The decaying plants make acids, the water keeps out oxygen, and the bogs are always in cold areas, and all of that preserves anything in the bog. Bodies that were in peat bogs for thousands of years were in such perfect condition you could analyze their last meal.