Concrete block in the concrete pad in the basement

[Robert Huffman]

Can anyone tell me what is the purpose34743 of putting a concrete block in the basement concrete pad? You can see the gravel below through the block opening. See attached image.

[Trent Tarter]

It's to let Radon gas into the home.

[Egbert Jager]

DIY safe rough-in...?

[Jack Feldmann]

Usually its a rough in for a bath tub drain under there.