Complex sentence: George Frank built his second mansion in Corning, Iowa, beside the largest man-made lake in South-West Iowa, which George also designed; this second mansion was called Edgewood.
Split into simpler sentences: George Frank built his second mansion in Corning, Iowa. It was next to the largest man-made lake in South-West Iowa, which George also designed; the second mansion that George Frank built was called Edgewood.