The Magic School Bus on the Ocean Floor is the fifth book in the original book series.
Ms. Frizzle's class is doing a project on the ocean and learning about the kinds of animals and plants that live there. Of course, a field trip is in her plan book. With her dash and aplomb, she gathers her students, they board the bus, and off they go on an adventuresome trip to the ocean floor. All listen as she informs them about the geologic strata they pass, the flora and fauna at the various depths, the unique independence of the undersea world, and in turn, how its survival depends upon humankind's actions on land.