The seed is "-123775873255737467" and the cave system is centered at -800, -1050; the landscape gives every indication of what lies beneath:
(the lava visible is a near-surface lava lake, not lava at y=11)
Here are some screenshots from inside; the typical description of big caves as "Swiss cheese" isn't really accurate; this is more like a sponge (MCEdit shows that some chunks actually have more air than solid blocks between the surface and lava layer, and there are no ravines in the main area):
Watch your step; it is a long way down through multiple holes like this:
To be specific, this is the seed for my first main world, where I explored all of the caves in an area measuring 2200x4000 blocks and the cave system in question was by far the biggest and densest single cave system I found; here is a map of that world I made with Unmined to show how big it is relative to other cave systems (at y=20 so denser caves stand out better); the cave system is in the upper-left, followed by a close-up at layers 62 and 20. Even with the latter, which only shows 10 layers (11-20), the main part of the cave system appears as a nearly solid mass:
As seen using Rei's Minimap after I fully explored it (very formidable unless you are very good at exploring caves, not that it fazes me):
There's also multiple deposits of diamond ore in the cave system, along with all of the other usual ores (not telling you the locations so you can find them yourself; there are more not shown, including parts of the veins here, and a vein or two hidden behind other ores):
Yes; I actually started the world in 1.5.1, though cave generation hadn't been changed since at least Beta 1.0*, so it works with any release version.
*Not kidding! I decompiled the jar and found the cave generator, the same code as 1.6.4, (mp.class for the curious, use Java Decompiler to view it, the relevant lines are near the bottom, where you have nextInt(40) and nextInt(15), changed to 15 and 7 in 1.7.x), although I couldn't find it in Alpha but the last change mentioned in the Wiki was InfDev, "caves now clustered like Swiss cheese"; the changes made in 1.7 make caves more like the way they were before. (oh, and I don't believe their remark made for Beta 1.2 for an instant; there is no connection at all between cave and ore generation).
Also of interest, here are maps of the cave system in Unmined at layers 60 and 30; you can see on the right that much of the lower half is so dense that there are no recognizable tunnels; the dense core measures about 80x128 blocks:
Wrong, cave generation is the same regardless of the version you use prior to 1.7; 1.6.4 is just the version that most people would likely use, being the most recent non-1.7+ version.
Just try it out for yourself if you can't believe me; go to edit profile and select 1.6.4 from the "use version" list.
So the 404 challenge is still partially possible now?
I looked at that seed before to see just how large the cave system was (pretty large, although not any larger than many big cave systems before 1.7), but the caves in a newly generated world were considerably different from a world download of the original 404 map, my guess is that the game handled the world seed differently; for comparison, by changing a few numbers back, I can get the exact same cave generation in 1.7 as in 1.6, independent of other changes to world generation.
Also of interest, while not what I'd consider to be one cave system, and nowhere near the density, the same seed has an enormous complex of caves and abandoned mineshafts from around -150, -150 extending northeastwards to around 250, -500, more than 500 blocks straight-line distance, with more caves to the west-northwest of the second point, including a stronghold near 0, -550:
(and yes, I explored all of that; I also found the stronghold by caving, actually the second one in that world I found that way, instead of using Eyes of Ender; this made me think that this was why I didn't get the End achievement (had to find it using Eyes of Ender), but that was a bug in 1.5)
The seed is "-123775873255737467" and the cave system is centered at -800, -1050; the landscape gives every indication of what lies beneath:
(the lava visible is a near-surface lava lake, not lava at y=11)
Here are some screenshots from inside; the typical description of big caves as "Swiss cheese" isn't really accurate; this is more like a sponge (MCEdit shows that some chunks actually have more air than solid blocks between the surface and lava layer, and there are no ravines in the main area):
Watch your step; it is a long way down through multiple holes like this:
To be specific, this is the seed for my first main world, where I explored all of the caves in an area measuring 2200x4000 blocks and the cave system in question was by far the biggest and densest single cave system I found; here is a map of that world I made with Unmined to show how big it is relative to other cave systems (at y=20 so denser caves stand out better); the cave system is in the upper-left, followed by a close-up at layers 62 and 20. Even with the latter, which only shows 10 layers (11-20), the main part of the cave system appears as a nearly solid mass:
As seen using Rei's Minimap after I fully explored it (very formidable unless you are very good at exploring caves, not that it fazes me):
There's also multiple deposits of diamond ore in the cave system, along with all of the other usual ores (not telling you the locations so you can find them yourself; there are more not shown, including parts of the veins here, and a vein or two hidden behind other ores):
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?
Yes; I actually started the world in 1.5.1, though cave generation hadn't been changed since at least Beta 1.0*, so it works with any release version.
*Not kidding! I decompiled the jar and found the cave generator, the same code as 1.6.4, (mp.class for the curious, use Java Decompiler to view it, the relevant lines are near the bottom, where you have nextInt(40) and nextInt(15), changed to 15 and 7 in 1.7.x), although I couldn't find it in Alpha but the last change mentioned in the Wiki was InfDev, "caves now clustered like Swiss cheese"; the changes made in 1.7 make caves more like the way they were before. (oh, and I don't believe their remark made for Beta 1.2 for an instant; there is no connection at all between cave and ore generation).
Also of interest, here are maps of the cave system in Unmined at layers 60 and 30; you can see on the right that much of the lower half is so dense that there are no recognizable tunnels; the dense core measures about 80x128 blocks:
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?
Edit - You even spawn next to a village? NICE! Looks like venice :3
Wrong, cave generation is the same regardless of the version you use prior to 1.7; 1.6.4 is just the version that most people would likely use, being the most recent non-1.7+ version.
Just try it out for yourself if you can't believe me; go to edit profile and select 1.6.4 from the "use version" list.
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?
I looked at that seed before to see just how large the cave system was (pretty large, although not any larger than many big cave systems before 1.7), but the caves in a newly generated world were considerably different from a world download of the original 404 map, my guess is that the game handled the world seed differently; for comparison, by changing a few numbers back, I can get the exact same cave generation in 1.7 as in 1.6, independent of other changes to world generation.
Also of interest, while not what I'd consider to be one cave system, and nowhere near the density, the same seed has an enormous complex of caves and abandoned mineshafts from around -150, -150 extending northeastwards to around 250, -500, more than 500 blocks straight-line distance, with more caves to the west-northwest of the second point, including a stronghold near 0, -550:
(and yes, I explored all of that; I also found the stronghold by caving, actually the second one in that world I found that way, instead of using Eyes of Ender; this made me think that this was why I didn't get the End achievement (had to find it using Eyes of Ender), but that was a bug in 1.5)
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?