Java Edition 18w09a

18w09a is the twenty-third snapshot released for Java Edition 1.13, which added coral blocks and underwater ruins.

Blocks

 * Coral Block
 * Comes in 5 colors: blue, pink, purple, red, yellow
 * Turns into dead coral when placed out of the water.
 * Dead coral cannot be revived.
 * Only obtainable in the creative inventory.

World generation

 * Ocean ruins
 * Come in many different shapes and sizes.
 * Cold ruins can be found in cold and frozen ocean biomes regardless of depth.
 * Warm ruins can be found in warm, lukewarm, and deep lukewarm ocean biomes.
 * Can generate alone or as part of a big ruined village.
 * Can also generate out of the water, slightly underground or slightly above sea level.

Mobs

 * Phantoms
 * No longer spawn in the End.
 * Will now despawn during daytime.

Command format

 * Tags
 * Added the vanilla block and item tag.
 * The  tag now includes chests.

General

 * Optimizations
 * Optimized particle rendering slightly

Video
Video made by slicedlime:

Trivia

 * Unlike most of the first-of-the-week snapshots, this was released on a Thursday instead of Wednesday, similar to the snapshots before 14w26a.