Minecraft wiki entry for Nether_reactor, associated text: If those conditions were met, tapping the core would activate the reactor, sending the client message "Active!". Immediately after activation, a massive structure of netherrack with multiple rooms, referred to as a Nether spire,[1] was generated around the reactor. The bottom room, which contains the reactor, replaced all blocks other than those of the reactor with air, while blocks in the upper levels remained intact.