Minecraft wiki entry for Acquiring_a_conduit, associated text: The only way to break the monument structure with elder guardians in the way is to detonate TNT. To do this effectively, place a gravity block such as sand on top of the TNT. When it ignites, it turns into an entity, causing the gravity block to fall inside it. When the TNT explodes, it destroys some of the prismarine and leaves some items. Without the aid of a gravity block, TNT damages mobs without destroying underwater blocks.