bit stuck! don't know where to connect my bathtub drain...

got stuck installing a new bathtub in the basement. I'm puzzled about the drain. where will it go? there is a big hole underneath it full of gravel but it don't seem to have any piping there to connect my bath drain. there's no where to connect its drain coz all the piping available are higher than the bath trap drain. that big hole, can it be use as a draining system. please help! need to finish this ASAP coz i'll be starting work next week.
many thanks...
derique of Calgary

If you live on top of a steep hill with extremely well-drained soil in a mild climate, that might work, but I doubt it would be up to any code I know of.

If i can't use that big hole as a drain for my basement bathtub... what else can i do?

You have to find the sewer main/

Have you dug that hole out to try to find the end of the sewer line or better yet, a capped off tub trap?

90% odds that hole full of gravel has a pipe in it. Find it.