Complex sentence: The park also contains a second volcano, Chato, whose crater contains a lagoon, it is also called Cerro Chato (literally Mount Chato) as it has been inactive for around 3500 years--coinciding with the creation and growth of Arenal itself.
Split into simpler sentences: The park also contains a second volcano, Chato, whose crater contains a lagoon. It is also called Cerro Chato (literally Mount Chato) as it has been inactive for around 3500 years--coinciding with the creation and growth of Arenal itself.