ON THIS WIKI
A Hydra Lair is a structure similar in size and shape to a Large Hollow Hill that has had approximately 1/3 of its surface cut off. Hydra Lairs, unlike hollow hills, contain stalactites made of mostly rock (ore stalactites are present, but rare) and a small amount of rock stalagmites without chests or ore. Each Hydra Lair also contains one Hydra that generates with the world. Each Hydra Lair contains materials such as Redstone, Lapis Lazuli, Iron, Gold and even the occasional Diamond, but are devoid of spawners and loot chests. However, to access any of these resources without being constantly assaulted, the Hydra must be defeated first.
This poses a challenge as the Hydra has a large quantity of health as well as dangerous attacks such as its fire breath. However, conquering a Hydra Lair can be a rewarding challenge, as doing so yields 6-10 Fiery Blood for use in powerful tools, 16-32 Hydra Chops which make a good temporary food source (heals 9 hunger units per chop as opposed to 4 from any other cooked meat) as well as access to the Hydra Lair's minerals.
Hydra Lairs are found in Swamp and Fire Swamp biomes, with the latter being their more common location due to their innate fiery nature. Hydra Lairs can be located easily using a Magic Map and appear as a 2D sprite of one of the Hydra's heads.