'''''The Twenty-One Balloons''''' (sometimes stylized as ''The 21 Balloons'') is a children's comedy-adventure novel by William Pene Du Bois. Set in 1883, it concerns a schoolteacher/adventurer, William Waterman Sherman, who decides to get away from it all by building a hot-air balloon capable of supporting a small house and setting off on a leisurely round-the-world vacation. Less than a week into his journey, the balloon is forced to crash-land on the island of Krakatoa, where Sherman discovers that a group of eccentric billionaires have created their own tiny society based around food and the shared wealth of a vast diamond mine hidden at the base of the volcano. Sherman becomes a permanent guest of the island, sharing both their secret and a portion of the diamond mines. Unfortunately, he has arrived barely a week before Krakatoa famously blows its top...

!!''The Twenty-One Balloons'' contains examples of

* AllBalloonsHaveHelium: Subverted. The mechanics and practicality of the various balloon inventions in the book are very accurately and carefully described. Some of them might even work in the real world!
* AlphabeticalThemeNaming: All the Krakatoans have given up their real names and are known only by initials: father Mr. A, mother Mrs. A, children A-1 and A-2, all the way down to T.
* {{Balloonacy}}: Happens to a small child during Sherman's homecoming celebration.
* CoolAirship: Sherman's balloon-house is described in exquisite detail: perfectly engineered to be operated by one person for an indefinite amount of time, stocked with lightweight furniture and trimmed with balsa wood. Even his mattress is made of balloon silk and filled with helium, so that it floats up to the ceiling for daytime storage.
** Equally cool but not quite as much fun is [[spoiler: the Krakatoan escape platform]]
* EccentricMillionaire: The inhabitants of Krakatoa have chosen to exist apart from the rest of society, under a "restaurant government" complete with its own twenty-day calendar, with no responsibilities other than creating exotic and incredible luxuries for their own consumption.
* GildedCage: Sherman is told on arrival that he is now a permanent guests of Krakatoa. Subverted in that, once he sees the diamond mines, he is quickly overwhelmed with the same fever as the other Krakatoans and no longer ''wants'' to leave.
** Inverted: the lure of the diamonds also keeps the Krakatoans themselves from leaving, in spite of the fact that they are now trapped on an isolated, inhospitable island where all their wealth is worthless.
* GentlemanAdventurer: Sherman primarily, but many of the Krakatoans have a streak of it in them.
* {{Greed}}: Of a benign sort. The Krakatoans are essentially trapped by their own greed, unable to leave the island without the thought of its vast wealth haunting them, but they're more than ready to share their wealth with Sherman.
* HereWeGoAgain: At the end of his story, Sherman plans to rebuild his balloon and continue his original adventure.
* TheMunchausen: [[spoiler: Subverted. At the very end, just as his audience begins to express doubt, Professor Sherman produces the diamonds.]]
* NuclearFamily: All the Krakatoan families were selected to fit into this trope, in order to insure genetic diversity. All families have a mother, a father, and a boy and a girl within a particular age range.
* OneLetterName: The Krakatoans all take alphabetical names from A to T. Sherman is discouraged from doing the same, as being called "U" would lead to pronoun confusion.
* SteamPunk: Many of the island's inventions fit right in with steampunk sensibilities.
* SteampunkGadgeteers: The entire M family.
* ThreateningShark: A school of them appears under the professor as his balloon is on the verge of crashing into the sea.
* VeryLooselyBasedOnATrueStory: Krakatoa is a real volcanic island that did explosively erupt in 1883, briefly encircling the entire planet with its debris.
* WorthlessYellowRocks: The diamonds are completely worthless on Krakatoa, where everyone is equally wealthy and there is nothing to buy--and taking the diamonds ''off'' the island would render them just as worthless everywhere else.