Steve joins forces with ropes expert Aldo Kane and Saudi Arabian geologists Mahmoud Al-Shanti and Hala Alwagdani to explore a volcanic underworld in search of the longest lava tube in the Arabian Peninsula. Geological evidence suggests that at least 400km of tunnels lie beneath the Harrat Khaybar lava field, subterranean passageways that could contain bones and artefacts from ancient civilisations. Only 25km of these lava tubes have ever been explored - until now