Mineshaft Overgrowth – for Minecraft Java 1.21.11

What it does:
- Overrides vanilla structure set: data/minecraft/worldgen/structure_set/mineshafts.json
- Increases mineshaft placement frequency from ~0.004 to 0.50 (legacy_type_3) with spacing 1 / separation 0.

Expected result:
- Mineshafts become extremely common and often overlap/intersect, producing a "practically endless" underground labyrinth feel.

Warnings:
- This is intentionally heavy. Worldgen can become slower and some areas may look chaotic.
- Use on a NEW world or explore NEW chunks to see the effect.

Tweaks:
- Lower frequency if you want less insanity (try 0.10, 0.05, 0.02).
- Keep spacing=1 / separation=0 if you want maximum overlap potential.
