Green rooftops seem like a modern innovation but have in fact been around for hundreds of years. Sod roofs are popular in Iceland and Scandinavia, which have a long tradition of using sod on farm houses and buildings. Mud was used to protect homes, and the vegetation that appeared naturally was found to be an effective insulator. The ancient Hanging Gardens of Babylon may be the first evidence of green rooftops. Around 600 B.C., Babylonian king Nebuchadnezzar II is thought to have created a series of lush garden terraces that ascended to the sky. In the 19th century, architects began to incorporate greenery onto flat rooftops. Green rooftops began to catch on in Germany in the 1970s, when existing sewage systems couldn't handle heavy rains.