The mythology of Ireland was originally passed down orally, but much of it was eventually written down by Irish monks, who Christianized and modified it to an extent. This large body of work is often split into three overlapping cycles: the Mythological Cycle, the Ulster Cycle, and the Fenian Cycle. The first cycle is a pseudo-history that describes how Ireland, its people and its society came to be. The second cycle tells of the lives and deaths of Ulaidh heroes such as Cúchulainn. The third cycle tells of the exploits of Fionn mac Cumhaill and the Fianna. There are also a number of tales that do not fit into these cycles – this includes the "immrama" and "echtrai", which are tales of voyages to the 'Otherworld'.