Minecraft wiki entry for Defeating_a_monument, associated text: A conduit can be set up in a 3 block high undrained room for up to 31 blocks worth of Conduit Power merely by placing a slime block, honey block or block of TNT on the floor one block diagonally in from the corner, placing the conduit on top and breaking the block. This removes any worries about air and helps speed up breaking through the prismarine if that is necessary.