DARPA's OFFensive Swarm-Enabled Tactics (OFFSET) program wants swarms of 250 collaborative autonomous systems providing critical capabilities to ground units in urban areas where challenges such as tall buildings, tight spaces, and limited sight lines constrain essential communications, sensing, maneuverability, and autonomous operations. Each of the five core sprints emphasizes one of the key OFFSET thrust areas – swarm tactics, swarm autonomy, human-swarm teaming, virtual environment, and physical testbed – to ultimately enable cross-cutting breakthroughs in swarm systems capabilities. DARPA is now running the fourth of the five sprints. This will be the virtual environment.