Procedural Content Generation in Games (PCG) is a thriving field of research and application. Recent presented examples range from levels, stories and race tracks to complete rulesets for games. However, there is not much research to date on procedural 3D modeling of caves, and similar enclosed natural spaces. In this paper, we present a modular pipeline to procedurally generate underground caves in realtime, to be used as part of larger landscapes in game worlds. We propose a three step approach, which can be fully implemented using General-Purpose Computing on Graphics Processing (GPGPU) technology: 1) an L-System to emulate the expanded cracks and passages which form cave structures in nature, 2) a noise-perturbed metaball approach for virtual 3D carving, and 3) a rendering component for isosurface extraction of the modeled voxel data, and further mesh enhancement through shader programming. We demonstrate how the interaction between these components produce results comparable to real world caves, and show that the solution is viable for video game environments. For this, we present the findings of a user study we conducted among indie-game developers and players, using our results.