This article breaks down everything you need to know about X-Ray texture packs for version 1.12.2.
Unlike a mod that injects code into the game, an X-Ray (or resource pack) exploits how Minecraft renders transparent blocks. By modifying simple PNG image files, the pack makes most solid blocks (stone, dirt, gravel, netherrack) completely or partially invisible. Only specific, valuable blocks (ores, chests, spawners) retain their full textures.
To disable: Simply move the pack back to the left side or click "Default."
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