If you are digging through solid ground and run into non-natural blocks, don't dig through them. If your curiosity is too great, you could at least perform the courtesy of repairing the hole afterward. If you don't have the blocks and the blocks are not retrievable when broken, don't do it. This goes doubly so for tunnels in the nether carefully crafted and decorated to create a safe passage. If I ever find someone leaving holes in stuff they didn't make, you can expect to find a red gift with white ribbon next to a pressure plate next time you return to your house.