I had a survival world starting on a island. But I was lucky enough to survive... had an amethyst geode connected to a ravine below my base... found moss blocks, dripstone, lots of materials and bamboo... So yeah, in the insane 1.18 update, I found a deep cave with a mineshaft. Then I found that the layer changes wasn't fully coded, because there was still the old lava layer. But I did not check everything for this evidence, because my world was already deleted. Wish I had better luck... and while I have not met this site yet, the evidence screenshot wasn't taken. Sorry Mojang!
MC-245350
1.18 update did not remove the old lava layer
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If you've upgraded an old world from before 1.18, the existing lava layer would stay and not be removed, that's intended.
There are supposed to be larger lava lakes above the lower lava layer, these are called "lava aquifers".