Underground Jungle

The Underground Jungle is a tangle of small mud chambers packed together with almost no straight line through it. Water fills half of them, thorny bushes clog the gaps, and the whole thing runs from partway down the Underground layer through the entire Cavern layer until it stops at the Underworld. It sits directly beneath the surface Jungle, on the side of the world opposite the Dungeon and the Snow biome.

It also spawns the ordinary Underground and Cavern enemy list on top of its own, so you are handling two enemy pools at once in terrain where you can rarely see more than a few tiles ahead. That combination, rather than any single enemy, is what makes it the hardest place in pre-Hardmode.

• Thorny bushes damage you on contact and grow thick enough to fill a corridor. Clearing them with a pickaxe as you go is slower and much cheaper than pushing through.
• Bee Hives hang overhead. Break one hive block and bees pour out; break the Larva inside and you have summoned Queen Bee wherever you happen to be standing.
• Water pools are deep and connect to each other. Dropping into one without breath gear puts you in an underwater fight against things that swim better than you do.

Bring light you can throw, a hook, and something that clears a room rather than one target at a time. Vision is the real constraint down here, so a Shine or Night Owl potion does more for your survival than an extra damage buff would.

Living Mahogany Trees, Jungle Shrines and the occasional Underground Cabin are scattered through it, and the shrines and the hollow trees both hold Ivy Chestes, which is the main reason to explore before Hardmode. Jungle Spores, Stingers and Vines come out of the walls and the wildlife and feed the entire early jungle equipment set. Depending on world size, three to seven Bee Hives generate, so a world with one badly placed hive still has spares.

A Jungle Temple generates somewhere in every Underground Jungle, walled in Lihzahrd Brick. You will almost certainly find it long before you can do anything with it. The door needs a Temple Key that Plantera drops, and the brick itself shrugs off every pickaxe until Golem gives up a Picksaw. Mark it on your map and walk away.

The biome unlocks in stages rather than all at once, which is worth planning your trips around.

• The moment Hardmode starts, Chlorophyte Ore begins forming in mud in the Cavern layer, and the evil biome gains the ability to convert jungle grass.
• After your first mechanical boss, Life Fruit starts growing on jungle grass. Each one raises your maximum health by 5, up to a ceiling of 500.
• Once all three mechanical bosses are dead, Plantera's Bulbs appear. Breaking one summons the boss immediately, so do not swing at a bulb you stumbled across while mining.

Every enemy here can also drop a Jungle Key in Hardmode. Farming it is easier before Plantera dies, since her death strengthens enemies across the world on Expert and Master.

The Corruption and Crimson chew through the surface Jungle quickly once Hardmode spread is running, but the Underground Jungle is vast and mostly mud, and mud is immune to the Hallow outright. Chlorophyte helps as well, since it blocks conversion nearby and turns dirt back into mud around itself. If you are choosing one place to defend and one place to put your Jungle house, both answers are down here.