Occurrence
SRTM GeoTIFFs often have elevation data embedded in the image file (e.g from http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp).
Discover can extract this information, creating a grid that can be
contoured and manipulated by the Surfaces menu (accessed via the Discover>Surfaces menu option).
Solution
Simply use File>Open>Grid Image .
Alternatively, download SRTM data in an Arcinfo ASCII format, which can also be opened directly from the File>Open>Grid Image.