Two-dimensional heightfields are the most common data structure used for storing and rendering of terrain in offline rendering and especially real-time computer graphics. By its very nature, a 2D heightfield cannot store terrain structures with multiple vertical layers such as overhangs and caves. This restriction is lifted if a volumetric data structure is chosen in place of a 2D heightfield. However, the workflow of manual modelling and editing of volumetric terrain usually involves a large number of minor edits and adjustments and is very time consuming. Therefore, I propose to use three-dimensional curve-based primitives to efficiently model prominent, large scale terrain features and present techniques for volumetric generation of a complete terrain surface from the sparse input data by means of diffusion-based algorithms. By combining an efficient, feature-based toolset with a volumetric terrain representation, the modelling workflow is accelerated and simplified while offering the full artistic freedom of volumetric terrain.