We introduce UNDERWORLDS, a novel lightweight framework for cascading spatio-temporal situation assessment in robotics. UNDERWORLDS allows programmers to represent the robot's environment as real-time distributed data structures, containing both scene graphs (for representation of 3D geometries) and timelines (for representation of temporal events). UNDERWORLDS supports cascading representations: the environment is viewed as a set of worlds that can each have different spatial and temporal granularities, and may inherit from each other. UNDERWORLDS also provides a set of high-level client libraries and tools to introspect and manipulate the environment models. This article presents the design and architecture of this open-source tool, and explores some applications, along with examples of use.