I'm gonna make this quick- Make a pre-1.7 world, generate land, find cave, bring the world to 1.8. If you didn't generate too much land, you'll still get 1.8 stuff and you get your cave.
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My avatar-Madison Gotha. She knows various weapons, she's a mage/sage, and is somewhat immortal. Madison has the capability of taming and communicating monsters, a genetic trait passed down by her father. 26 physically, 1400-something mentally, and a Lofty Peakian/Zenithian mix. Doesn't really like being called an elf and hates being called human..
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You can also use a mod I made that perfectly replicates pre-1.7 cave generation (the "old caves" mod here; and yes, it works on 1.8; I even have a version that makes double the old cave generation). Note that ravines are somewhat different since 1.7 but they have the same frequency and everything (the change was just in the way they curve around due to changing an array from 128 to 256 elements, changing the sequence of random values used); mineshafts are also 40% as common since 1.7 which IMO is a more sensible rate when you are far from spawn (they become less common, down to none, as you move towards the origin, 0, 0; in 1.6.4 they occur at the rate of one every 100 chunks, one per 250 since 1.7, past 1,280 blocks from 0, 0; of course, they become a bit too rare close to the origin so in my own world mods I have them generate at the 1.7+ rate everywhere).
And yes, I think it is very lame that they don't allow any sort of configuration in the customized settings (aside from making the landscape itself form "caves" through overhangs and such ("Caver's Delight"), but I wouldn't want to play on such a world, I want more or less normal terrain) - especially since Superflat has allowed said configuration of other structures for years - yet they couldn't be bothered to even include that! Particularly for caves it is very easy to change how big/dense they are - the mod I mentioned was made by editing the raw bytecode, no MCP, etc required.