Test the limits of your endurance in one of a unique ultramarathon: a 200-mile challenge set almost entirely underground in the UK’s longest foot tunnel. Athletes tackle approximately 100 out-and-backs (a total of 200 passes) through the Combe Down Tunnel in Bath, Somerset, covering mostly flat terrain in relentless darkness and sensory isolation. Between 11pm and 5am, you'll contend with pitch-black conditions; outside those hours, low-level lighting offers limited respite. There’s a checkpoint at the tunnel’s south end, stocked with water, coke, and light snacks such as bars, cake, and gels—remember to bring your own mug or container. A gruelling 55-hour time limit puts both physical and mental strength to the ultimate test in what may be the greatest mileage race 99% below ground.