How do you make a ‘deep’ pit?

I’m trying to make a root cellar, which I need a deep pit for. I don’t seem to be able to dig that far though. Do I need to use the ‘dig downstairs’ button to make that?

as far as I’m aware you need some kind of proper shovel with digging 2, the stick is only enough for a shallow pit

I made a regular pit, more than just a shallow pit. My entrenching tool has 3 digging quality.

dig the same tile twice

That gave me a regular pit, not a deep pit. Then the game said I couldn’t do it again.
I need a deep pit. Deep.
1st shallow pit
2nd pit (I got this)
3rd deep pit

The construction menu list requirement for root cellar is, pit, I know the description says deep pit is needed, but pit works.