Maintenance for the week of March 16:
• PC/Mac: No maintenance – March 16
• ESO Store and Account System for maintenance – March 18, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 UTC)
That's still there? I encountered that when I first started playing at launch. Yes, it's not supposed to be there, but you'll find them throughout the world.
They are mystery boxes that contain the secrets to world domination, becoming the emperor, having the powers of a god, and all that wonderful stuff. The only way to open them, however, is to complete the entire game 8 times over while hopping on one leg, then asking ZOS to shoot the box with a star from the heavens, then you must peer inside the box, and there is a 99% chance that it is empty, and then you must start the process over again. If you do not complete these steps on the dot and miss even the slightest quest, or skip any parts, then the powers of the box will consume you and the world!
In all seriousness though, I have actually seen some of these boxes from time to time, and there seems to be less and less of them as time moves on. Perhaps, as @Sleep mentioned, repairing might help, but I think it might just be ZOS fixing some glitched objects in the world from their end.