Pipelines are generally laid underground because temperature is less variable. Because pipelines are usually metal, this helps to reduce the expansion and shrinkage that can occur with weather changes. However, in some cases it is necessary to cross a valley or a river on a pipeline bridge. Pipelines for centralized heating systems are often laid on the ground or overhead. Pipelines for petroleum running through permafrost areas as Trans-Alaska-Pipeline are often run overhead in order to avoid melting the frozen ground by hot petroleum which would result in sinking the pipeline in the ground.