Many older buildings in certain areas of Boston are supported by wooden piles driven into the area's fill; these piles remain sound if submerged in water, but are subject to dry rot if exposed to air for long periods. Groundwater levels have been dropping, to varying degrees, in many areas of the city, due in part to an increase in the amount of rainwater discharged directly into sewers rather than absorbed by the ground. A city agency, the Boston Groundwater Trust, coordinates monitoring of groundwater levels throughout the city via a network of public and private monitoring wells. However, Boston's drinking water supply, from the Quabbin and Wachusett Reservoirs to the west, is one of the very few in the country so pure as to satisfy federal water quality standards without filtration.

Lots of older buildings are supported by what?