These mineshafts are common in Mesa/Badlands biomes. They have been added since the 1.10 update. They are the same as regular mineshafts, except that the wooden structures are made of dark oak instead of regular oak.
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Just figured out that when I'd turn 8 years old, the 23th of November 2011, my parents could have offered me Minecraft as a birthday gift. Sigh...
Fun but frustrating platformer. Saves me of boredom when I don't feel like playing Minecraft. Knowledge of 8-bit old-school videogames is optional tho'.
I’m glad you posted this question because I learned something new. I have only once found a mesa biome and although it did have a surface mineshaft (it was the first mineshaft I had ever found) I was a brand new player at the time and didn’t pay attention to the kind of wood it had or realize that mesa biomes or dark wood mineshafts are rare.
"Rare" (specific to Mesa -> now-"Badlands") but the most- visible and accessible. The other thing Updated with that is that they became what I call "Gold Biomes" (the Dark Oak, Wood Planks show up the glittery yellow Ore the best, by contrast).
They're practically "rainbow AbMS's," due to the sheer-and-Random (enough, when you're in there) wall textures. I've just found 4 of them within easy walking distance of one-another, which was the most, but it's not-unusual to have 3 within the same-range (typically with the Dirt floor room either at the Surface or just-below) quite often, roughly around Sea Level, explore happily!