Danger in the Deep

The 'dangerous' mines are the regular 120 levels remixed with far nastier monsters. You unlock them late: during Mr. Qi's "Danger In The Deep" special order (Walnut Room, Ginger Island), or afterwards at will via the Shrine of Challenge on mine level 120. Reach level 100 during the Qi order, or level 120 with the shrine active, and it pops.

Treat it like a proper expedition: galaxy-tier weapon, plenty of food, bombs for the ore-heavy floors, and use the lift to break it into trips. The new monsters hit hard but drop generously - it's the best radioactive ore source in the game, so the trip pays for itself.