I've noticed this a few times now, the dungeon "ClassicSpider" is generating a large pillar of mossy cobblestone above the surface and placing the spawner at the top rather than in the dungeon.
example below.
other world gen mods present if you can't reproduce this:
Underground biomes
geographicraft(aka climate control)
biomes o plenty and/or realistic terrain gen.
I've noticed this a few times now, the dungeon "ClassicSpider" is generating a large pillar of mossy cobblestone above the surface and placing the spawner at the top rather than in the dungeon.
example below.
other world gen mods present if you can't reproduce this: Underground biomes geographicraft(aka climate control)
biomes o plenty and/or realistic terrain gen.