Advanced World Generation and Cave Content, also known as simply "Cave Content," is a mod that generates underground caves in the game. While it may generate ancient technology, it does not allow players to explore underground caves directly. Instead, it generates underground caves in various terrains similar to grasslands, deserts, forests, and swamps.
Under default map settings, at least one underground terrain will be generated, such as a glowing fruit area in the swamp biome where there are glowing plants and a few large tentacles and tendrils. There is also a stone shrimp area where there are some stone shrimp and stone structures. The coal mining area contains coal ore that can be mined with a pickaxe. The reed area has denser reed coverage than aboveground terrain and may have tendrils or spiders (occasionally no spiders).
There are rabbit dwelling areas where ten or more rabbit houses may appear on a brown-colored ground biome. The ruins area includes statues that can be broken with a pickaxe for ancient materials related to magic. The maze area appears like an actual maze with white spider nests and chests, along with underground bosses like the ancient rhinoceros (usually found near ruins areas), but there have been reports of finding chests within these maze areas.
Finally, there are monkey dwelling areas where monkeys can be found along with banana trees (an update for this mod has been mentioned that it can generate these areas). However, personally I have not encountered any such instances yet.
Please note that these descriptions are based on my knowledge of the mod as an AI language model and may not be an accurate representation of all possible scenarios or configurations within the game itself.