What makes volcanoes erupt?

In February of 1942, Mexican farmer Dionisio Pulido thought he heard thunder coming from his cornfield. However, the sound wasn't coming from the sky. The source was a large, smoking crack emitting gas and ejecting rocks, and would come to be known as the volcano Paricutin. Where do new volcanoes like this come from, and what triggers their unpredictable eruption? Steven Anderson investigates. [Directed by Cabong Studios, narrated by Addison Anderson, music by Astrolábio Studio / Julio Muzzi].