There aren't that many seeds being posted for 1.6.4 and older versions (this seed should also work back to 1.2.1 for the world spawn point, and the cave itself back to Beta 1.8) but I think they still deserve to be mentioned, especially when it comes to the underground generation. The seed "-6923329256941061233" will spawn you right on top of a cave system that can only be described as enormous, the biggest cave system I've ever seen this close to (much less under) spawn.
Spawn is located at the yellow player marker; the first image shows the underground below sea level, second below y=20 (dense caves stand out much better at this layer, I made these using Unmined) and an AMIDST map of the seed. There is also another large cave system to the northeast:
You can find sunlight reaching all the way down to lava level where a ravine cuts through the cave, as well as some huge open spaces:
Here is what it looks like under spawn using an x-ray texture pack:
For another perspective on how big this cave system is, here is the result of analyzing an 8x8 chunk area between layers 11-62 in MCEdit - 29.6% of all blocks within that area were air; one chunk contains 13,312 blocks between layers 11-52 so this is nearly 19 chunks with nothing but air within this area, or an average of more than 15 layers of air per chunk (this does not include the entire cave, just the densest area):
Also, while versions before 1.6.4 are known for their big cave systems, caves like this one are extremely rare as shown on this map of one of my worlds (seed "-123775873255737467", also in 1.6.4), which has a similarly large and dense cave near the upper-left (much further from spawn, at -800, -1050), which for a long time I considered to be the largest cave ever found and the majority of worlds that I've looked at do not have anything close to these caves within several thousand blocks from spawn; caves are much bigger on average though than in 1.7 and later versions:
Note that there are 65,535 other seeds which will produce this same cave since Java's Random only uses the lower 48 bits of the world seed so by adding or subtracting 2^48 (e.g. "-6923610731917771889" and "-6923047781964350577" have the same lower 48 bits) you can get a different world with the same caves and almost anything else besides biomes and landmasses, even biome-dependent features if the biome in a particular spot is unchanged. I found this cave using a program that I wrote to search for big cave systems near spawn, which scanned hundreds of thousands of seeds and listed the largest caves found, then used AMIDST to find a "same lower 48 bits" seed that placed spawn directly over it (spawn is usually around x = -256 to 256 and around z = 200 due to a bias in the code that finds a suitable spawn point; spawning at 0, 0 or somewhere far away occurs if it can't find a valid "spawn" biome). It is also possible to get the same caves in 1.7+ if a mod is used; unfortunately, Customized only lets you enable or disable caves (only a couple variables need to be changed so would be very easy to implement).
There aren't that many seeds being posted for 1.6.4 and older versions (this seed should also work back to 1.2.1 for the world spawn point, and the cave itself back to Beta 1.8) but I think they still deserve to be mentioned, especially when it comes to the underground generation. The seed "-6923329256941061233" will spawn you right on top of a cave system that can only be described as enormous, the biggest cave system I've ever seen this close to (much less under) spawn.
Spawn is located at the yellow player marker; the first image shows the underground below sea level, second below y=20 (dense caves stand out much better at this layer, I made these using Unmined) and an AMIDST map of the seed. There is also another large cave system to the northeast:
You can find sunlight reaching all the way down to lava level where a ravine cuts through the cave, as well as some huge open spaces:
Here is what it looks like under spawn using an x-ray texture pack:
For another perspective on how big this cave system is, here is the result of analyzing an 8x8 chunk area between layers 11-62 in MCEdit - 29.6% of all blocks within that area were air; one chunk contains 13,312 blocks between layers 11-52 so this is nearly 19 chunks with nothing but air within this area, or an average of more than 15 layers of air per chunk (this does not include the entire cave, just the densest area):
Also, while versions before 1.6.4 are known for their big cave systems, caves like this one are extremely rare as shown on this map of one of my worlds (seed "-123775873255737467", also in 1.6.4), which has a similarly large and dense cave near the upper-left (much further from spawn, at -800, -1050), which for a long time I considered to be the largest cave ever found and the majority of worlds that I've looked at do not have anything close to these caves within several thousand blocks from spawn; caves are much bigger on average though than in 1.7 and later versions:
Note that there are 65,535 other seeds which will produce this same cave since Java's Random only uses the lower 48 bits of the world seed so by adding or subtracting 2^48 (e.g. "-6923610731917771889" and "-6923047781964350577" have the same lower 48 bits) you can get a different world with the same caves and almost anything else besides biomes and landmasses, even biome-dependent features if the biome in a particular spot is unchanged. I found this cave using a program that I wrote to search for big cave systems near spawn, which scanned hundreds of thousands of seeds and listed the largest caves found, then used AMIDST to find a "same lower 48 bits" seed that placed spawn directly over it (spawn is usually around x = -256 to 256 and around z = 200 due to a bias in the code that finds a suitable spawn point; spawning at 0, 0 or somewhere far away occurs if it can't find a valid "spawn" biome). It is also possible to get the same caves in 1.7+ if a mod is used; unfortunately, Customized only lets you enable or disable caves (only a couple variables need to be changed so would be very easy to implement).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
Awww, I wanted to make a map.
Great find though, this is pretty awesome! Perfect for mining.