Complex sentence: It has been rumored that the outlaw Joaquin Murrieta once had a cave/hideout, that he used while on the run, however nobody has been able to find it, so it is just an "rural" legend.
Split into simpler sentences: It has been rumored that the outlaw Joaquin Murrieta once had a cave/hideout in the area that he used while on the run. However, nobody has been able to find it, so it remains a "rural" legend.