This is another potentially serious issue. A lateral line is a line running from your water main into your home. These pipes can get old, or be affected by tree roots, animal or seismic activity. Whatever the case, it won't take long for a leak like this to get expensive as the water drains into the ground underneath your home. This is another case where the advice of a professional plumber is crucial to finding and fixing the problem. This is an issue that can't be ignored.