I have just installed a one piece tub/shower unit in my basement. I had to break out about 15 inches of concrete slab from the box that was originally there to accommodate a tub. The hole is completely under the tub which which will be framed into the walls on 3 sides. Should the opening in my slab that contains all the plumbing be filled or covered somehow or can it just be left open under the tub?

Thanks,

I would at least fill it with coarse sand.