凹
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!