Monolith

Monoliths were glitched areas of terrain that happened in the late versions of Infdev and early Alpha eras of the game. These monoliths would cause the terrain to abruptly generate up to the height limit, with natural grass block and ore generation. Unlike regular chunks errors, they were visible to anyone playing in the world.

Generation
Monoliths would generate in seemingly random locations throughout the world. They tended to generate around flattish terrain. They were caused by an error in the Perlin noise generator. This huge terrain bug would be surrounded by regular terrain, and sometimes even have regular terrain inside of it. Under the monolith, there would be a water-filled hole down to bedrock.

Structure
The monoliths would cause the terrain to abruptly generate up to the height limit, with natural grass block and ore generation. They could theoretically generate infinitely upward, being stopped only by the height limit, which at this point in the game's development, was 128 blocks. The area below the monoliths was completely hollow, except for water generating at sea level and a layer of bedrock at the bottom, making the normal terrain seem like inverse monoliths. It is possible to find small crevices in large monoliths, where normal terrain was generated.

Issues
Monoliths are an issue, and due to their removal, can be considered already fixed.

Trivia

 * By setting "Biome Scale Weight" to negative values in old customized worlds, they could generate  from snapshot 14w17a for 1.8 to snapshot 18w05a for 1.13, but with the removal of the "Customized" world type altogether in snapshot 18w06a for 1.13, this can no longer be recreated.
 * Monoliths can once again be generated in 1.16+ using customized worlds, however the settings to actually produce them are unknown.
 * It is not known if the reworked terrain generation in 1.18 is capable of harboring monoliths.
 * In between Java Edition Infdev 20100624 and Java Edition Infdev 20100629 the bedrock layer directly underneath the monolith was raised to sea level, making the void easily accessible underneath the monolith.
 * Monolith-like terrain bugs still occur in Minecraft Bedrock Edition.

Gallery
https://static.wikia.nocookie.net/minecraft_gamepedia/images/5/51/BE_Terrain_Breakdown_-_Monolith.png/revision/latest/scale-to-width-down/120?cb=20211214050428| Monolith-like structures generating beyond X/Z ±2,812,333 in Bedrock Edition.