I would imagine you will find a cast box that the tub sits in. That is an unusual arrangement. I have never seen anything recessed into a slab like that. you might be able to pull the tub, fill the box and install a new tub in the same place at a normal level.

Or you could install a guard rail at the tub to prevent accidental entry!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.