Ambience

Ambience (sometimes called cave noises) is a set of sounds that occasionally plays when the player is in close proximity to a dark cave. They were made by C418, who also composed Minecraft's music. Ambient sounds can be played in any dimension, including the Nether and the End and can be heard in both singleplayer and multiplayer. Despite their name, they are not sounds that one would expect to hear inside of a cave, such as falling pebbles or dripping water, but rather supernatural noises frequently likened to demonic roars, vehicles, ringing bells, or instruments.

For ambient sound to play, the game requires a space of only 1x1x1 blocks (but the noises will play more likely in larger spaces) positioned 10 - 20 blocks away from the player in any direction, and completely filled with a light level less than eight without any lighting from the sky (the ambience will more likely play from the direction of lower levels because of a lower light level). The space does not need to be sealed off - a sound can even play from a space that the player can actually see - nor does the player need to be underground to hear it. The sounds can also come from player-created spaces, even if that space is a box above the clouds.

The Sounds
Ambient sounds are found in the resources/newsound/ambient/cave from the .minecraft folder. They randomly play in a normal, lower or higher pitch. There are 13 sounds (Cave 1 - 13) in total.

History
As of snapshot 12w07a, the map format was updated to the Anvil format, which now allows ambience to be heard in Multiplayer.

Ambience was known to play only in 3x3x3 spaces or larger, but at least now they can be heard playing in 1x1x1 or larger spaces.

Bugs

 * Sometimes it doesn't matter how far the player is from the space.
 * As of snapshot 12w07a, building height is increased, but dynamic lighting does not work above the old 128 block limit without blocks, causing ambience to play.

Trivia

 * Ambience is not an indicator of nearby ore, underground structures, mobs, etc. Following them can still lead the player to such things indirectly, since they emanate from unlit caves.
 * Due to the unsettling atmosphere of ambience, these sounds can be frightening to young players, and annoying to older players. However, they do not represent any immediate danger. They only play in deep darkness. If you do not want to hear the sounds, light the caves you find (which is recommended, regardless, unless playing on Peaceful difficulty).
 * Cave sound 8 resembles a stretched blaze sound. Although it sounds like there is one right around the corner, rest assured; one should never encounter a blaze outside of the Nether unless it is spawned via spawn egg or third-party mod. Also, the sounds were implemented a while before blazes were.
 * Ambience can play anywhere in The End.

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