The Hydra Lair is a structure similar in size and shape of a Large Hollow Hill that has had approximately a third of its surface demolished. The main feature of the Hydra Lair is the Hydra, which fills most of the area. Hydra Lairs also contain materials such as Redstone, Lapis, Iron, Gold and even occasionally Diamond. However, to access any of these resources without being constantly assaulted, the Hydra must be defeated first.
Hydra Lairs are found in the Fire Swamp Biome. Hydra Lairs can be located easily using a Magic Map and appear as a sprite of one of the Hydra's heads.
If you are playing according to the progression system, the air in the fire swamp will be thick with flying cinders and ashes. You will be unable to enter the fire swamp without catching fire. Defeating the Minoshroom will allow you to enter the area safely.