The neighborhood is built upon a former tidal marsh, a part of a larger project of the filling of Boston’s Back Bay (north and west of Washington Street) and South Bay (south and east of Washington Street), from the 1830s to the 1870s. Fill was brought in by trains from large trenches of gravel excavated in Needham, Massachusetts. The South End was filled and developed first, before the Back Bay which was mostly built after the American Civil War. Nineteenth century technology did not allow for driving steel piles into bedrock and instead a system of submerged timbers provided an understructure for most South End buildings. Recent decreases in underground water levels have caused damage to some wood pilings by exposing them to air. A series of monitoring wells have been drilled and the water level is now checked, and can be adjusted by the introduction of water.