There is way more to these hills that meets the eye (or the picture above). If you survey the relatively flat ground of the Twilight Forest, you may notice these dome-shaped hills. Beneath the stone shell, the hills are hollow, filled with stone and ore stalactites and stalagmites.
Hollow Hills appear as white mounds on a Magic Map; the size of the mound on the Magic Map corresponds to the Hill's size. Likewise, the size of the Hill corresponds to the variety, quantity and rarity of the treasure within. Sometimes, Hollow Hills have Caves running through them, allowing for them to be noticed from the surface or discovered while caving.
These are the most common landmark in the Twilight Forest, and they come in three different sizes. The Larger the hill, the more valuable ores and treasures, but also more challenging monsters.
Small hills have mostly Coal, Iron, and occasionally Redstone stalactites, as well as a few small Glowstone ones. Monsters include Spiders, Zombies, Skeletons, Silverfish, Swarm Spiders, Redcap Goblins and Kobolds. There are typically 2 treasure chests, which usually have fairly common items, such as Buckets, Ore Magnets or Torches, but may rarely have a Diamond or Steeleaf or two.
Medium hills are about 60 blocks in diameter, and 25+ blocks tall in the center. The interiors are more open inside, and thus more dangerous. Coal, Iron, Redstone, Gold, and Lapis can be found among the stalactites and rarely you may find some diamonds and emeralds. Up to 6 treasure chests can be nestled among the stalagmites. Monsters include Zombies, Skeletons, Spiders, Creepers, Swarm Spiders, Redcap Goblins, Kobolds, and Fire Beetles. Treasures are more valuable and could include Ironwood Ingot, as well as unique and rare treasures like the Transformation Powder, Moonworm Queen, Uncrafting Table, and the Peacock Feather Fan.
The largest hollow hills cover an area around 112 blocks in diameter, giving them an interior volume of over 175,000 blocks. Every type of ore can be found in these cathedral-like caverns. There are up to 12 treasure chests, containing the most valuable treasures. Naturally, one will find the most dangerous monsters here, including Skeletons, Spiders, Cave Spiders, Creepers, Endermen, Redcap Goblins, Redcap Sappers, Wraiths, Witches, Fire Beetles, Slime Beetles, and Pinch Beetles. Treasures of the most value can be found amongst these hills such as the rare Naga Scales and Steeleaf.
Hollow hill treasure chests are the only place to find Ore Magnet. Additionally, large hollow hills are the only place to find the Wraiths and Redcap Sappers.