Yes, that's retarded NCsoft crapping the game. This is a box, which normally contains another small area (instances, rooms, boss areas, quest stuff, video sequence stuff.... and so on... You can add such boxes using Unreal Editor.
The first time I saw it was on Fafurion Client (1st release). I thought they fixed it in later versions, but now I know I was wrong.... Good Job, NC!
It normally should be flagged as invisible, or be placed under the map, where players can not see it. But hey... degenerated NCsoft doesn't care about the L2-World and it's exploration aspect anyway.....so they do shit like this in live releases.
You can use a modded camera to swing through walls, if you want to see what's inside, or you can start dev-mode. get with "open 22_23" into that particular map, then using "camerawalking mode=on" and "ghost" to fly around and through walls with WASD or ARROW KEYS.
Have fun! ;)
PS: NCsoft became a real piece of shit - real NCsoft - not NCwest. NCwest was degenerated right when it was formed...