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.

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 can place and destroy blocks and interact with objects outside the world border.
 * Falling sand and gravel will become item entities.

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

"/worldborder center [x] [z]" sets the center of the area inside the world border. "x" and "z" are the x and z coordinates.

"/worldborder set [diameter] [time]" 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
In Beta 1.7.3 and before, the world stopped after 32 million blocks from the origin, but weird terrain called the Far Lands occurred after about 12 million blocks.

In Beta 1.8 and after, but before 14w17a, the blocks were not solid after 30 million blocks from the origin, and the player would fall through.

See Far Lands for all the different effects in previous versions.

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.