Generally the bottoms are not completely glazed. My guess is that the markings are permanent.

But if they are glazed there, try rubbing alcohol or hair spray.

I would try a Mr. Clean magic eraser. However, these are a little abrasive, so you would have to way your options on that one: faint scratches or marker?

My son wrote on my dry erase board in permanent marker (Sharpie). A friend said hand sanitizer, such as Purell would take it off. It DID!!! Maybe it was the alcohol in it, I don't know, but I was amazed that it worked. May as well try it!

Tell us if it worked!