Like corns on the toes a deep callus under the foot is caused by too much pressure on the area, usually under one of more of the metatarsal heads. In response to the excess pressure, the callus builds up and can eventually become so thick that it is painful. If the higher pressure is over a smaller area and quite focused, then a corn tends to develop whereas a callus develops when the pressure is over a wider area.
A podiatrist can easily remove a deep callus by skilful debridement, but that does not necessarily get rid of it as the pressure that caused it in the first place is still there, so it does tend to come back eventually. The only way to get rid of a deep callus on the bottom of the foot is to get rid of the reason that it is there and that is the excessive pressure. Firstly the reason for that higher pressure needs to be identified. It could be poor fitting shoes or the use of a shoe with too high a heel. It could be for biomechanical reasons that result in one or more of the metatarsal bones being too mobile, so that when weightbearing it gets pushed up out of the way and then there is too much pressure on the other metatarsal bones resulting in the callus. It could be due to a deformity from an old fracture. There are multiple reasons for that excessive pressure on an area to cause a deep callus.
Secondly, once the reason for the higher pressure is identified, then steps can be taken to reduce it to help get rid of the callus. This could be as simple as changing footwear or as complicated as reconstructive surgery to change the weightbearing pattern where the callus is. It could mean the use of foot orthotics to improve the foot biomechanics and make the metatarsal bones more stable as a well as keep the pressure of the painful areas. Sometimes none of these strategies might work and the primary option then is a regular debridement by a podiatrist to keep it under control.
If you are concerned abut a deep plantar callus and want to know your options to get rid of it, then consult a podiatrist for advice on the options.