You can use a trapdoor trick to glitch your Minecraft character into a crawling position and crawl through 1 block tall spaces. You can also use water to get into a swimming position to acieve the same results. The Minecraft character can run, jump, swim and even fly using elytras. It only makes sense that the same player can also crawl on the ground. The GoProne mods adds an option to make your player go into crawn mode so that you can crawl through 1 block tall spaces without relying on glitches. Once you install the mod, head over to your control settings and assign a keybind to the crawn action. Once you do that, you can press the shortcut key to make the player go into a crawling position.

This is a Forge Mod and download links are given below.

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GoProne Mod