I agree, I don't think they go deeper than the OP's one inch cap.
That is not accurate. Last spring I syphoned off my Black Diamond Cap that was 1-1.5" deep. That left a 2" layer of MTS. There were snails coming up out of the dirt.
I can not say the were coming from thew bottom, nor do I know how deep into the dirt they were. They were without question deep than the sand cap an in said dirt.