World border

The world border is the edge of the world, introduced in 14w17a. Without commands, it is at 30 million blocks in every direction from the origin. However, with commands, it can be made smaller.

The world border appears as many diagonal light blue lines alternating with clear lines. If the border is expanding, the lines are green, and if the border is shrinking, the lines are red. The "shimmery" texture was made by Ryan Holtz.

Effects

 * Mobs and players cannot move outside the world border.
 * Mobs and players cannot move more than a block if they are already outside the world border, except for players in spectator mode. This is exactly like what would happen if the player was in solid stone, except for visibility.
 * Shot arrows and thrown snowballs can still pass through the border, but item entities and fishing rod lines cannot.
 * Mobs and players suffocate as if they were in a solid block if they are outside the world border.
 * Light is not affected by the world border.
 * Players cannot place and destroy blocks and interact with objects outside the world border, although they were able to in 14w17a.
 * Falling sand and gravel will become item entities.
 * Explosions, pistons, water and lava work normally outside of the border and can alter terrain as they normally do. This means that sticky pistons and slime blocks can be used to retrieve blocks outside of the border.

Commands
To change where the world border is, there are two commands:

sets the center of the area inside the world border. "x" and "z" are the x and z coordinates. "~" (tilde) can be used as a relative coordinate.

sets the border to a square region with the specified diameter and the center in the previous command. If "time" is specified, the border grows or shrinks in the specified number of seconds. If "time" is not specified, the world border changes immediately. To remove the world border, set "diameter" to 30000000 (30 million).

History
See World boundary for all the different effects of the world boundary prior to the introduction of the world border.

Trivia

 * The effect of the world border is always in full block increments, even if it looks like it is between blocks.
 * Even though the lines don't show above y=256, the border is still effective.
 * With the command to change the border over time, the border will still grow or shrink even if the game is paused. The animation also happens even if the game is paused.
 * In the Nether, the world border's dimensions are the same as the Overworld's.