Regular old cleavage and side boob are quite common, but underboob is very rare. The amount of underboob wouldn't have to be too explicit, just a little. Like a dress with a cleavage window beneath the breast instead of above it.
Most Helpful Guy
I haven't seen this a lot, but sometimes if a really busty girl is wearing a bikini and it's centered, some of the bottom of her breasts will show.1