A causeway is a raised path, railway or road across an expanse of low ground, wetlands or water. It is different from a bridge in that it has little or no opening underneath. Instead, it consists of a crest with embankments on either side. It is typically made of compacted earth, sand and rocks. In most cases, causeways are made by humans to connect different dry land areas to each other. Such connections can also be made up of a combination of causeway and bridge segments. Sometimes, a causeway can serve several purposes simultaneously. In addition to the passage it provides, the bulk of its structure may be intended to function as a dam or dike.