Terraria At what layer will mimics spawn

You can go exploring some corrupted/crimson areas (randomly picked at world generation) to destroy the shadow orbs/crimson heart waiting for you at the bottom of chasms.

Each shadow orb/crimson heart destroyed will provide you some loot, and make a meteorite land on the world. Once the meteorite has fallen, go find the site, and mine the meteorite ore. This will allow you to craft some meteorite gear and weapons.

Once you've destroyed three shadow orbs/crimson heart, it will make the Eater of Worlds/Brain of Cthulhu spawn, allowing you to fight two different bosses.

Note: The shadow orbs are protected behind some ebonstone/crimstone, which you will not be able to mine with your current pickaxe. Just buy some dynamite from the Demolitionist and use it to access the shadow orb. Or you can buy purification powder from the Dryad, which turns the ebonstone/crimstone into the easily destroyable standard stone.

I set up two mushroom gardens to test this:

One: -200 feet, torches every few steps under the one block of mud, about 4 mushroom grass planted at the same time.

Two: -800 feet two mushroom grass planted at the same time, one with a torch underneath, one with no torch.

Result: In garden one the tall mushrooms grew all over the place at about the same time. In garden two the tall mushroom grew under the torchless portion first.

Conclusion: torches don't really matter, just be at negative depth, give about 20-25 blocks of space above. Above all, wait. The tall mushrooms take a while to show up.