The South End is composed of landfill. Where your home stands used to be part of the back bay / Charles river. Boston needed more space, so they filled in the land and sold it off for housing. You can see some of the landfill when the street is torn up for construction - often you'll see sand and shells as some of the original fill for the area.
But landfill makes a terrible foundation for a building. To provide structural support, wooden pilings were driven into the ground, and your foundation stones were laid on those pilings. These wooden pilings support your building. (Usually along the 4 outer walls)
The health of these wooden pilings is dependent on a delicate balance of water and air. The parts of the piling above the groundwater table stay nice and dry and are fine. The parts of the pilings that are below groundwater are also fine - the pilings do quite well as long as the parts under water stay under water.
Herein lies the problem
Several years ago the city starting paying more attention to the groundwater issue. The Groundwater Trust was formed, and they have put several dozen monitoring wells throughout the South End and Back Bay to monitor the groundwater levels.
We encourage you to visit the Groundwater Trust web site to read more about groundwater in Boston, groundwater monitoring and recharging efforts, and keep up on the latest groundwater news.