Outcropping in Stockholm is common, and it is not unusual to see buildings constructed directly on rock. However, there is much variety. For example, on the edges of Lake Mälaren’s outlet to the Baltic Sea, around which old Stockholm is built, there are significant alluvial deposits in which many old and impressive buildings have their foundations. Most often, these consist of substantial wooden piles driven into the alluvial deposits. Taken togetherand considering the already-well-developed underground infrastructure in Stockholm, with which any new developments must co-exist-the foregoing presents engineers with a variety of geotechnical challenges.