The Cracked Floor

Now and then the machine's floor cracks where a hole should be. Let the hammer strike it: the crack runs further up the same column and grows deeper. The third strike breaks it open.
How to find it, step by step
- In arcs 1-3 a floor tile with NO hole appears on the second farthest row, with fine cracks in it.
- Stand in its column so the automatic strike hits that tile when its row reaches the line (first hit = The Cracked Hole).
- A seam runs up the column and the crack reappears further up, one stage deeper. Hit it again, then a third time.
- The third hit collapses the floor: 30 seconds in THE UNDERGROUND, then a treasure chest (1 of 3 rewards).
The crack is in the machine's FLOOR: an empty backing tile on the second farthest row, with no hole (no mole can stand on it). Only the hammer's automatic strike counts. The tile cracks in 3 stages: hairlines, bigger cracks with crumbs, then shattered with a warm glow from below. After each hit a seam creeps up the same column and the crack reappears further up. The cracks match the floor of your cabinet: earth tiles split organically, metal panels fracture in straight lines, neon and circuit glass breaks in spokes with sparks. A crack you miss is gone and comes back 18 rows later (up to 3 misses).
Reward
The way down.
Tips
In-game hint
Some floor tiles look... tired.