"Blows Its Top" is the first episode of season two and the fourteenth episode overall of the animated children's series The Magic School Bus. It originally aired on September 9th, 1995.
The class is trying to piece together a giant globe, with the exception of Dorothy Ann, who is reading the instructions of the globe. Suddenly, Ms. Frizzle flies out of a model volcano and the globe eventually falls apart. When D.A. tells Carlos that the globe was missing one more piece, the two end up arguing as Carlos chides D.A. for researching rather than putting the globe together and D.A. retorting that research would've helped put the globe together. Ms. Frizzle says that it was missing an island "so new, it hasn't even been discovered yet!", adding that the earth changes all the time, even under everyone's feet. Carlos eventually gives Ms. Frizzle an idea to discover the island and name it. So Ms. Frizzle decides to take the class out for a field trip.
The class ride the bus, which Ms. Frizzle transforms into a two propeller-ed airplane and Carlos and D.A. decide to try and find this island, the two arguing over what it'll be named. While they think up names for the island, the bus comes across ashes floating in the sky, which blackens out the bus windows as Liz accidentally lands onto a lever, causing the bus to plunge to the ocean.
Ms. Frizzle transforms the bus into a seaplane and lands it where the island should be. As the seaplane bus continues to ride through the water, D.A. decides to name the would-be island "Empirical Island". The bus comes to a stop in the middle of the ocean and the class exit the bus to find that there is no shore in sight. Ms. Frizzle tells the class that they are "a tad early", adding that the class should "get out there and explore" right before inflating a raft in the ocean. Carlos hops onto the raft and splashes water with enough force to cause D.A. to lose her book bag. She pleads with Ms. Frizzle to let her go after it, but Carlos would rather take the raft and look for the island. Wanting to keep "as much water between him and the bottom of the ocean as possible," Arnold goes with Carlos, with Liz tagging along, while everyone else gets on the bus.
As Carlos sets off to look for the island, the bus transforms into a submarine and dives underwater to explore the deep as D.A. looks for her book bag. They come across what appears to be an underwater mountain, which D.A. states that it is impossible for her to explain since she doesn't have her books. She gets a glimpse of her bag through the window and the bus swims down to the bottom of the ocean to find a sunken ship. The class exit the bus wearing their diving suits and D.A.'s bag falls inside the ship. D.A. swims inside the ship to retrieve her bag, only to find that it is caught in the tentacle of a squid, which swims out of the ship. Just then, an earthquake precipitates, and D.A. grabs her bag off of the squid's tentacle, only to get inked and lose her bag again. Little does she know, gas bubbles had sent it to the surface, where Carlos and Arnold find it, the former thinking the contents will help locate the island.
Back underwater, D.A., oblivious to the whereabouts of her book bag, continues searching for it as the class swim along with her down to the bottom of the ocean, in which the temperature drops into. The class come across what appears to be smoke pipes and D.A. figures that something below them is heating them since she doesn't have her books to explain what they are. When the class come across what appears to be an underwater canyon, Ms. Frizzle takes them back into the bus and under the canyon.
Meanwhile, Carlos is reading one of D.A.'s books and comes across the information the earth's surface is made out of layered crust-like plates of rocks, which moves slowly and sometimes, one plate jams onto another. Furthermore, when the plate slips the earth moves.
Back underwater, Ms. Frizzle compares the world to a toasted bowling ball with a crust, telling the rest of the class that they are between the edge of two pieces of crust. In order to squeeze between the narrow space of the underwater rock, the teacher transforms the bus so that it is flat enough to squeeze in between the rock and the underwater surface as it gets hot.
When the bus makes it inside, the class discovers that the crust is melting. Ms. Frizzle tells the class that it gets hotter inside the earth's depth and informs that magma is melted rocks. D.A. thinks about the exploration the class has experienced and finally surmises that the class is inside the heart of an underwater volcano.
Meanwhile, Carlos continues to read D.A.'s books and Arnold is convinced that Carlos is starting to sound just like D.A. as Carlos tells him that the two are floating on top of an underwater volcano.
Back into the volcano, the magma rises to the top and D.A. informs the others that when all the molted rock shoots up out of the volcano and hits the air, it cools, which causes it to harden to new rock and build on top of the volcano, thus creating a new island. The bus then comes to a stop on top of the volcano since something is plugging up the volcano. So the class dress into their magma gear and exit the bus to give the rock on top of the volcano a push. When the class struggle to push the rock, Ms. Frizzle uses the bus to crack open the rock and the volcano finally erupts, making a new island as Carlos and Arnold witness it from the surface of the ocean.
As the volcano erupts, the rest of the class land onto the raft with Carlos and Arnold and Ms. Frizzle also joins in along with the bus. The class witness Liz on top of the island and claims the discovery. It was because when D.A. landed hard on the raft, it bounced up Liz and it sent the paddle slamming into her and sent her flying toward that island. Carlos then offers D.A. her book bag, saying that he was glad to have it to read the interesting research inside the books. D.A. also says that she was glad she didn't have her books for a change to learn more about things from experience instead. D.A. finally suggest that she and Carlos should write a book about the discovery of the island (which she names "Lizard Island") When Carlos and D.A. argue over whether the book should be by either "Carlos and D.A." or "D.A. and Carlos", Ms. Frizzle states that "unlike the earth, some things never change".
- Arnold and Tim's new voice actors, Danny Tamberelli and Andre Ottely-Lorant, debut in this episode, replacing Amos Crawley and Max Beckford.
- This is the first episode where D.A. uses her memory instead of her books for the field trip.