凹 - Ao1 - Concave

Yes, really! 凹 means concave! And that’s not all. Guess how you write convex?

凸 - Tu1 - Convex (although I have to point out that technically, the character 凸 is not convex)

I swear I’m not making this up! I only looked this up because our new house has a 凹洞 - Ao1 Dong4 - Concave Hole — Pit in the backyard.

Jenny also tells me that you can describe a bumpy road as 凹凸不平 - Ao1 Tu1 Bu4 Ping2 - Concave Convex Not Flat!