flat
Explanation
The Joke
The comic is titled "Alternative Non-Spherical Earth Theories" and presents a table of alternative Earth shapes, each with a "Theory," "Pro," and "Con" column. The shapes include: Mobius Earth (explains why you can walk in one direction and end up where you started, but difficult to explain the ribbon-like surface or why "up" stays up), Half-Flat Earth (good excuse for never traveling to other countries, but compromises the "pleasing purity" of saying the Earth is round), Bowl-Shaped Earth (possible -- nobody has actually been to Alaska, but hard to explain why the ocean has never poured milk over us), Anaximander Map Earth (geography lessons are easier to memorize, but American cheese is harder to obtain), Slightly Lumpy Oblate Spheroid Earth (technically correct, but so nerdy that nobody wants to hear about it at parties), Hollow Earth (the inside giant is unharmed, but it frankly seems like it would be a problem), and Non-Existent Earth (can't be proven wrong by humans since they'd also be non-existent, but violates the principle that when you pinch yourself you feel it).
The comic is a parody of flat Earth theory that riffs on the idea that if you're going to reject mainstream science about the shape of the Earth, why stop at "flat"? There are infinitely many wrong shapes to choose from, and each has its own absurd advantages and disadvantages.
The Humor
The humor comes from treating obviously ridiculous theories with the same deadpan analytical framework one might use for a serious scientific comparison. The pro/con format gives each absurd idea a veneer of thoughtful consideration. Some of the best jokes are buried in the details: the Slightly Lumpy Oblate Spheroid entry is especially clever because it's actually the correct scientific description of Earth's shape, but it's listed alongside the absurd alternatives as if it's just another crackpot theory -- and its "con" is that it's too nerdy, not that it's wrong. The Non-Existent Earth entry provides a delightful philosophical paradox as the closer.
References
The Anaximander Map Earth references Anaximander, the ancient Greek philosopher who proposed that the Earth was a flat cylinder. The "Slightly Lumpy Oblate Spheroid" is indeed the most accurate description of Earth's actual shape -- it bulges at the equator and has various gravitational anomalies. The Hollow Earth theory references a real (if fringe) historical hypothesis that the Earth contains a hollow interior, sometimes said to contain civilizations or giants.