Underground is unlocked when candidate reaches level 12. Mobilize every Child you have as you battle your way to its deepest depths. Your foes will become ever more powerful as you near the very bottom, but you can earn the highest rarity of equipment and tons of Golds, among other rewards.
You may enter Underground 3 times per a day, but you have to reset the dungeon using Crystals to challenge it again.
You can choose between three different difficulties( Easy, Normal, or Hard). Depending on the difficulty, the rewards you earn changes. There's a total of 15 stations and each time you pass through 3 stations you earn reward at a separately established supply station. Childs’ HP does not recover after battles in Underground, but the fever gauge remains. Use your Childs' skills wisely.