Raised baseplate
From Brickipedia, the LEGO Wiki
A raised baseplate is a baseplate with raised sections, with or without studs on top. The first example is part 3947, which includes a flat section with the edge of a crater. Raised baseplates were used extensively in the 1990s and early 2000s and were often printed with pictures of different landscapes. LEGO have since stopped using them.
These elements are large plastic pieces that are stretched out in the third dimension to form various landscapes. Raised baseplates usually represent hills and feature recesses that can serve as basements or caves for the structure built on top of them. Studs can be found on sections on the ground level as well as on the top or intermediate levels of the landscape.
