Minecraft Dungeons has an array of levels for you to progress through in the game. While these missions are all available after you beat the game and play it on a higher difficulty, you may have missed some of the secret levels in the game. For example, in the Soggy Swamp, you can unlock the Soggy Caves, which has several puzzles in it you need to solve to receive the obsidian chest at the end. For Highblock Halls, you can unlock the secret level, Underhalls.
All you have to do is progress through the first portion of the Highblock Halls level until you reach the first staircase. The staircase you want to find should have a large chandelier above it, with two carpets on the upper floor. Make your way down the staircase, and at the base to it should be a stone wall with two shields on the wall. The shield on the right can sometimes be red, but they’re typically black and white. Approach the shield hanging on the left side of the wall, interact with it, and open the stonewall at the base of the stairs.
Proceed inside, and the map for Underhalls is waiting for you. Grab it, and the mission will now be unlocked for you for the rest of the game. There are no tricks or traps associated with this secret level.
Unlike the previous secret missions in Minecraft Dungeons, this one does not seem to rotate around the map or randomly show up. You can go about it the same method almost every time. The interactable shield appears to show up each time the level loads up, making it extremely easy to add to your collection of secret levels.