VATERITE is a microcrystalline form of calcium carbonate. It is rarely observed in nature, but has now been identified in a precipitate formed in Holkham Lake, Norfolk. The lake is nearly one mile long, is fed by springs at the south end and discharges over a weir towards the sea at the north. At the south end a narrow stretch, about 200 yards long, is separated from the main body of the lake by a low dam which maintains a slightly higher water level. In spring 1970, the water at the south end developed a marked milky opalescent appearance. Biological investigations showed no unusual species at either end of the lake, and ho evidence of pollution. Chemical and structural analyses identified the suspended material as chiefly calcium carbonate.