In the Introduction to Dorothy and the Wizard in Oz, L. Frank Baum good-naturedly complains that "The children won't let me stop telling tales of the Land of Oz. I know lots of other stories, and I hope to tell them, some time or another; but just now my loving tyrants won't allow me."
Every time he thinks he's getting out they pull him back in.
Our story begins in California. Dorothy and her Uncle Henry have survived their trip to Australia and are now on their way back to Kansas, but they've stopped off to visit some relatives. Since this is California and the year is 1908 (two years after the great quake that leveled San Francisco), the place is utterly plagued by earthquakes. Dorothy can't even ride the train to her cousins' ranch without being delayed by tremors.
Her cousin Zeb is waiting at the depot with a buggy drawn by the bony old horse Jim. They set out for the ranch, and you'll never guess what happens. Actually, you probably will: a natural disaster casts them into a fantastic realm of magic. Happens all the time when Dorothy is around.
This suggests an interesting possibility: rather than the gateway to Oz being located in a particular place, maybe certain people have the ability to reach Oz. Dorothy, the Wizard, Button-Bright, and perhaps some others simply have the Gift. Stress and fear seem to be the most common triggers -- Dorothy shifts to Oz in a cyclone, a storm at sea, and an earthquake. Button-Bright apparently does it unconsciously, "getting lost" without noticing. The Wizard's ability may be like Dorothy's -- unless he has a way to consciously control it.
Note that Dorothy's power is able to bring along nearby people and objects -- the Kansas cabin, a chicken coop, and now a horse and buggy. Because she can't control this ability, she can't really "target" where it takes her, either. Instead of materializing safely in Ozma's courtyard, she tends to wind up in odd places in and around Oz. Still, it beats getting killed by some natural catastrophe.
In this case, the trigger for Dorothy is an earthquake. It's fortunate that she and Zeb are in a remote part of rural California because the quake is insanely powerful, at least Richter 9 or worse. It doesn't just shake the ground, it actually splits open a gigantic rift miles deep, into which the buggy falls. Zeb, Dorothy, Jim the horse, and Dorothy's pet kitten Eureka go falling into the darkness, with only the canopy of the buggy to act as a parachute and prevent them from smashing to bits on landing.
They fall for a long time. Evidently the atmosphere down in the rift is quite dense, because they don't seem to be falling very quickly. Even allowing for the parachute effect of the buggy canopy, they should still be moving at close to terminal velocity -- and they aren't. Still, they descend a heck of a long way. Assuming a falling speed of only 100 kilometers per hour (half of terminal velocity above the surface of the Earth), and that what seems like a long time to Dorothy is only five minutes, they still descend a good eight kilometers. That's not enough to pass through the crust, but it's still deeper than any humans have ever gone.
Instead of very hot basalt, as one would expect to find five miles underground, they see a strange country inside a gigantic cave. The cave is lit by six multicolored miniature suns, and at the bottom is a city made of glass. For a moment there's some concern that they're going to land on one of the many sharp pointed glass spires of the city and be spitted like oysters en brochette.
The children make another interesting discovery as they fall: the animals can talk. This comes as a surprise to everyone except Dorothy, who is getting a bit jaded about magical happenings by now. Eureka the kitten is also permanently colored pink by the light of the miniature suns, so that in all subsequent books she is known as the Pink Kitten, capitalized.
Finally the buggy and its occupants comes down quite slowly and safely atop a big glass building in the city. The whole place is disturbingly silent, and the children wonder if it's abandoned, until finally a man appears to greet them. He's very handsome, very calm -- and he can walk on the air. Turns out everyone can walk on the air in this cavern; we never do find out why. What with the talking animals I'm going to go out on a limb and say it's magic.
The people of this strange underground country are the Mangaboos, and they are all extremely handsome -- yet creepy-looking. Dorothy can't quite put her finger on why. Apparently L. Frank Baum came up with the idea of the Uncanny Valley about a century before it became a common concept. (Quick summary: if you draw an imaginary graph of how realistic a face is and how attractive it is, the attractiveness generally rises as the realism increases -- until you reach a zone just before perfect realism, at which the attractiveness takes a nosedive before recovering. That region in which something is almost realistic, but not quite, is the Uncanny Valley.)
The Mangaboos are upset because Dorothy & co. were accompanied as they fell by a rain of stones (sorry, a Rain of Stones) which did a lot of damage to the glass houses of their town. They're looking for someone to blame, and the surface-dwellers are prime candidates. When Dorothy insists they're not to blame, the Mangaboos decide to take the interlopers to see the Sorcerer. And that's where we'll come in next time.