Board Thread:Episode Discussion/@comment-32598036-20160128010759/@comment-25396609-20160128222311

It's an even sided octogon, you can see two corners clearly in the bottom of the picture, and the bottom line is equidistant to both of them. The floor has one seam at each corner, therefore the lower line is one such seam.

Based on the angle shown of the scene, the upper line connects to the opposite corner, meaning it is another seam in the arena floor.