Minecraft wiki entry for Nether_Portal_(block), associated text: Much like water or lava, portal blocks cannot be broken by tools except in Creative. However, portal blocks can be destroyed by explosions (even weak ones) and can be replaced by placing water or lava sources in the same block (can also be done by dispensers). If any portal block is destroyed, the whole portal is deactivated as adjacent portal blocks are updated and detect that the portal structure is no longer complete.