This mod adds the infinite IKEA known as SCP 3008 to Minecraft. To enter the dimension, you just have to get lost in a suspicious furniture retail store store. These generate throughout your world. To exit, you just need to find an exit and go through it. But doing so is easier said than done as the exits move around, and they also avoid lost wanderers. Fortunately, someone has found a way to construct a device to catch an exit and hold it in place long enough for you to go through (by right-clicking). You can find the instruction booklet in the office section of the showroom, but the parts are scattered throughout the complex typically inside of chests and barrels.
