Tension is building between the Aztecs and the Spaniards, not to mention within Cortés's own ranks. Despite this, the conquistador rallies his troops to conquer Mexico.

In part seven of this episode on the ecosystems, the young students are on a field trip (a marsh trip to be exact), where they witness firsthand the destruction of this precious ecosystem. But this is not a new phenomenon: once upon a time in Polynesia...