Pls. Help! Stuck with my basement bathtub drain...

[Deriquelite]

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

 

[Frenchie]

Dig out the gravel, it was put there to keep the cement out during the pour. There's a pipe buried in there.