If the surface between a marked and an unmarked region is wrinkled, you can use the /adapt/smooth-register TUI command to smooth it.

> /adapt/smooth-register
marking register id/name  [] 0

  118586 cells marked after smoothing step 1
  119718 cells marked after smoothing step 2

The smoothing is accomplished by marking additional cells at the boundaries of the specified marked register. The resulting collection of cells (i.e. the cells in the original register and the newly marked cells) are added to a new register. The original register is preserved. You can thus see which cells have been added to the marked region using register operations.