In search of the longest cave in the world: exploring two huge cave systems on the Yucatán peninsula in Mexico: Sac Actun and Dos Ojos. For more than twenty years, many outstanding cave divers had tried to find a natural connection between the underground corridors, only after 20 years did they manage to do so – twice, but still none of the connections is possible to dive. Therefore, it is a great challenge in diving exploration of caves. The extreme sections of the Sac Actun (270km long) and Dos Ojos (79km long) systems are located only a dozen or so meters apart. If a natural underwater connection between them was found, the largest completely flooded and possible to dive cave system in the world would be created, with a total length of almost 350 km.