Java Edition 1.17 Public Overworld Surface Test

The 1.17 Public Overworld Surface Test (commonly known as the 1.17 POST or the POST) was a public test started by Mojang in collaboration with SicksonFSJoe, Earthcomputer, NeilCaffrei, BOINC, Minecraft@Home and many other coders and players. This was a test to check on how existing seeds's surfaces from before 20w46a would change in that snapshot for the 1.17 update. 
==Why the POST started==
The Public Overworld Surface Test was made to check on the noise changes on the surfaces of the Overworld between 1.16.3 and 1.17 snapshot 20w46a. This was because the new cave generation was built using a custom optimisation mix of the 3D Perlin noise generator, that has been used and optimised several times since the beginning of the game, and a more sophisticated OpenSimplex noise generator, that was slightly optimised for the game's caves by later rescaling and adding the slices to generate a sort-of fractal noise. OpenSimplex was kept to the 2D-sliced setting to be more complatible with Perlin noise when mixing between altitude levels in-game. 
The mixing was done by having any cave use OpenSimplex, and having the surface (y=62 being the limit for the surface) and the nearest blocks below it use Perlin. This was done by having naturally-generated blocks such as grass blocks, dirt and stone receive experimental receptors