Underboob is pretty great stuff since the abrupt change in lines shows off those curves against the straight slim ribs n' trunk.
More so clothes varieties which hold/press the breasts closer to the chest,creating shadows that add more depth.
And for every underboob we get,there's easily 10 or even 20 upperboob,low cuts,deep V-necks,and any other variety that shows a smooth flow.
We see plenty upperboob. We see plenty underboob too.
We don't see enough sideboob.
Depending on the medium,we see those more then underboob.
But they do instill the same uniqueness,more so when the lass is lying down with her ass up and pressing her chest in the surface,buldging out the sides to add more curvature.