A 200 mile ultramarathon held on a flat out-and-back course along a disused railway line incorporating the Combe Down Tunnel near Bath. The route is almost entirely on firm, level surfaces with minimal elevation gain, making the challenge primarily one of sustained distance rather than terrain. Lighting is required for the tunnel sections. Participants pass the central base area repeatedly, where aid and support are provided. Event logistics, navigation approach and time limits are defined by the organiser’s official guidance.