As spring thaws warmed the lakes and streams of northern Poland last week, Professor Zdzislaw Rajewski, director of Warsaw's archaeological museum, gathered students and laborers to resume a fascinating job that started more than 25 years ago: the excavation of Biskupin, a surprising pocket of ancient civilization wondrously preserved for 2,500 years under a Polish lake about 50 miles northeast of Poznan. Hidden beneath the waters are the remains of a thriving agricultural society that lived in the Iron Age, when the Greeks and other civilized Mediterranean peoples considered northern Europe a primeval prowling ground for savages.
What now ranks as...