Minecraft wiki entry for Defeating_a_bastion_remnant, associated text: There is always a magma cube spawner in each bastion remnant of the treasure room type. It is located close to the bottom of the large room, above a lava pool, guarding the ultimate treasure: a pile of gold blocks, and a chest. Most players would just easily disable the spawner by breaking it, as magma cubes spawn at any light level, torches won't do anything, and it takes a lot of time and blocks to fill up the entire 9×3x9 spawning area, enough time for the local piglins, piglin brutes and magma cubes to tear you into pieces. However, there exist a simple way to disable a magma cube spawner entirely and reliably, using as little as 9 blocks.