I thought I was digging out the last of the granite in a hole, but it turned out I was digging into the top of another cavern.
I thought about climbing down to get the ore that was inside, but then I was jumped by two zombies that tried to shove me down into the cavern (not a safe way to climb), and I changed my mind.
I threw them into the cavern and decided the ore down there could wait.
Then I dug out the rest of the floor/ceiling of granite so they couldn’t climb back up. That was a bit sadistic, but it made me feel safer, so I’m not beating myself up over it.