What's New in Minecraft 1.18 Experimental Snapshot 1?

CHANGES IN EXPERIMENTAL SNAPSHOT 2 COMPARED TO SNAPSHOT 1

Biome placement is a bit smoother and less noisy. Fewer microbiomes dotting the terrain. Biomes tend to be a bit larger and less fragmented.
Raised cloud level from 128 to 192
Renamed mountains (= old mountains biome) to extreme hills, and gravelly mountains to gravelly hills, to reduce risk of people getting confused when they use /locatebiome to find the new mountains and end up in the old mountains...
Beachier beaches! Beaches are generally wider. But also in some places there are no beaches at all, to provide some variation.
Sheep spawn in meadows. Also more rabbits and less donkeys
Blue orchid no longer spawns in meadows. The swamps were sad because this was their unique thing.
Swamps now generate properly. They generated in weird places before and were sad about that. Swamps are happier now.
Added missing biomes: Ice Spikes and Eroded Badlands
Underground biomes interfere less with surface biomes. They can still leak out of cave entrances sometimes though (because it looks kinda cool!)
Increased the height of some of the peaks. Now they can reach up to 220-260 or so, but rarely.
Slightly increased the chance of finding large areas with flat terrain
Meadows sometimes spawn a lone tree, often with a bee nest
Monsters spawn in the new mountain biomes
Emeralds and infested stone generate in the new mountain biomes
Made ore veins slightly larger and more frequent, since very few people seemed to actually find them in survival.
Structures now show up in the right biomes (mostly). Found the right amount of duct tape to put in the right place in the code.
Toned down the megacave entrances a little bit, and made them less likely to go all the way down to deepslate level. Getting to diamond zone was just a bit too easy.
Made cheese caves a little bit smaller on average (they can still be huge though!), and a bit less likely to intersect the surface
Reduced the chance of sand and gravel being placed in such a way that they immediately fall down on generation. Replaced with sandstone or stone in those cases. The falling blocks were messing up the terrain as well as performance. Still happens but not as often.
Zombie, Skeleton, Spider, and Cave-Spider spawners now spawn mobs up to block-light level 11. Although the mobs spawning in complete darkness did not affect the difficulty of vanilla spawners, we recognized that these spawners haven't been a real challenge for a while now. We hope this change will make conquering a spawner a more exciting challenge!
Spawners have a new "CustomSpawnRules" tag, which lets you override the spawn rules of the spawned mob. In the tag you can specify "BlockLightLimit" which indicates the highest block-light the spawners will spawn mobs at. This change is to enable map-makers to use spawners in a more customizable manner to spawn mobs in any light levels they choose!

CHANGES COMPARED TO CAVES & CLIFFS: PART I

New world height and depth
New mountain biomes (lofty peaks, snowcapped peaks, grove, meadow, and snowy slopes)
New cave generation (cheese, spaghetti, and noodle caves)
Local water levels and lava levels in the underground
New cave biomes generate naturally underground and inside mountains (lush caves, dripstone caves)
Natural variation in terrain shape and elevation, independently from biomes. For example, forests and deserts could form up on a hill without needing a special biome just for that.
New ore distribution (see this chart)
Large ore veins
Integrated mountains, caves, and cave entrances
Monsters will only spawn in complete darkness

HOW DO I GET THE EXPERIMENTAL SNAPSHOT?

NOTE: These snapshots are experimental! Some features may be significantly changed or even removed if needed to improve performance.

INSTALLATION

For a visual guide how to install the experimental snapshot, check out this nifty link with pretty pictures! Otherwise, just follow the steps below:

Download this zip file
Unpack the folder into your "versions" folder of your local Minecraft application data folder (see below if you are confused)
Create a new launch installation in the launcher and select the "pending 1.18_experimental-snapshot-4" version
Start the game and the remaining files will be downloaded
Play in a new world! This version is not compatible with other snapshots!

FINDING THE MINECRAFT APPLICATION DATA FOLDER

Windows: Press Win+R and type %appdata%\.minecraft and press Ok
Mac OS X: In Finder, in the Go menu, select "Go to Folder" and enter ~/Library/Application Support/minecraft
Linux: ~/.minecraft or /home/<your username>/.minecraft/

WHAT ABOUT THE PREVIOUS CAVES & CLIFFS PREVIEW DATAPACK?

These changes are too big for a datapack, that's why we're doing it in a separate experimental snapshot. So for now, the experimental snapshot serves the same purpose as the datapack did before.

CAN I OPEN OLD WORLDS IN THIS EXPERIMENTAL SNAPSHOT?

No, experimental snapshots can only open worlds that were created in the same version. Later on in the actual 1.18 release you will be able to open 1.17 worlds.

WHAT ABOUT BEDROCK?

An earlier version of experimental Caves & Cliffs world generation is available on Minecraft Bedrock. You can read about it here. We are working hard to get this new version of world generation to Bedrock betas soon.

HOW DO I GIVE FEEDBACK?

Use the feedback site or this Reddit post.

We are mostly interested in feedback about the new world generation overall, and what it is like to play in it. We are also looking for feedback on the updated mob spawning. We changed so that mobs only spawn in complete darkness in order to make it easier to spawn-proof the new larger caves.

New feature requests are not so useful at this point, since the scope of the Caves & Cliffs update is already large enough and we want to focus on finishing the features that we've already announced.

Note that we don’t use the bug tracker for experimental snapshots.

WHEN WILL THESE FEATURES SHOW UP IN THE NORMAL SNAPSHOTS?

1.18 snapshots will probably start sometime around September.

ORE DISTRIBUTION