As part of a new course on geology, Ms. Frizzle assigns her students to bring rocks to school. After most of the class fails to do the homework, she decides they will all go on a field trip to collect rocks. But this is no ordinary rockhounding expedition. With the help of the titular school bus, which turns into a steam shovel and then later grows a drill, they travel all the way through the Earth and back out the other side.
This book is notable for a few significant developments in the evolution of the series. It marks the first time that the bus transforms and carries the class on their entire adventure. That may seem like the basic premise of the series, but it was nonetheless not really present in At the Waterworks. This book also contains the debut of Phoebe, who is introduced as a "new kid." Finally, the book's fact-checking section features a fictional telephone conversation between a fault-finding kid and the book's author and artist, which is the most direct basis for the producer segments in the TV series.
In 1990, this book was featured in a Reading Rainbow episode, making it the first Magic School Bus book to be adapted for television. However, when The Magic School Bus became its own TV series, it never really did an adaptation of this book. The closest is the volcano episode, "Blows Its Top," which has a significantly different premise and storyline. On the other hand, this book was loosely adapted as a computer game, The Magic School Bus Explores Inside the Earth.
This book provides examples of:
- Butt-Monkey: Arnold's status as such starts to emerge around this point in the series. After the whole trip through the Earth, he's apparently the only student who failed to find a real rock.Ms. Frizzle: Arnold, that's not a rock. That's styrofoam.
Arnold: [thinking] Not again! - Chekhov's Volcano: When the bus comes out on the other side of the world, it ends up on a volcanic island. Downplayed, since they came out of the volcano's vent, meaning there's a narrative reason for it to be a volcano even if it doesn't actually erupt. That said, it erupts soon afterwards anyway.
- Characterization Marches On:
- In the TV series, one of Arnold's signature character traits is his love for collecting rocks. That trait is nowhere to be seen in this book, in which he tries to pass off styrofoam as a rock despite his classmates managing to find real rocks. The video game adaptation corrects this, with you, the player, finding rocks for Arnold's collection rather than a class collection that he doesn't care about.
- Not only does Dorothy Ann fail to do the homework, she is explicitly shown offering up a lame excuse. This is not very in character for someone who is later portrayed as a rather serious student.
- Distinction Without a Difference:Ms. Frizzle: First, we will dig through the Earth's crust. The top layer of the crust is soil.
Arnold: [thinking] It looks like dirt to me.
Ms. Frizzle: Dirt is another word for soil.
Arnold: [thinking] Oh, great! Now she can read my mind! - A Dog Ate My Homework: As an excuse for failing to do the homework, Dorothy Ann claims that she found a rock, but her dog ate it. John replies, "Your dog ate a rock?"
- Drill Tank: Inside the cave, the bus grows a drill and starts boring through the wall. It keeps going until they travel all the way through the Earth.
- Growling Gut: When the volcano starts rumbling, Phoebe asks Arnold if his stomach is making the noise.
- Is It Always Like This?: This is Phoebe's main schtick in this book. The other kids repeatedly tell her that she'll get used to it, and by the next book, she does indeed seem to have assimilated to Ms. Frizzle's class. Ultimately, the use of this trope provides the basis for Phoebe's "at my old school" catchphrase in the TV series, making her seem less assimilated in the long term.
- My Car Hates Me: When the volcano begins erupting, the bus naturally refuses to start.
- New Transfer Student: Phoebe is introduced as a new student in this book. If you've ever wondered why the TV series portrays her as a Naïve Newcomer despite her being part of the show from the beginning, this is why.
- No Name Given: The fact-checking section consists of a telephone conversation between "reader," "author," and "artist." But presumably, "author" and "artist" are fictionalized versions of Joanna Cole and Bruce Degen, and the real Cole and Degen did indeed voice those roles for the original audiobook.
- Rainbow Lite: When the bus goes up into a cloud at the end, it rides on a rainbow parachute. The five colors represented are red, orange, yellow, green, and blue.
- Series Continuity Error: The previous book ended with Ms. Frizzle announcing that they would be studying volcanoes next, apparently setting up this book. However, this book starts off with the kids having spent "almost a month" learning about animals' homes before the Friz declares that they're now switching to geology.
- Visual Pun: There's an illustration of a candy dispenser, which is labeled "rock candy" and has literal rocks inside of it.