I must have made a little mistake somewhere. When i am swimming through my water tunnel the walls are just "disapearing". Here is also a short videoclip http://www.file-upload.net/download-2515692/Bughelp.wmv.html Actually this is the first time i build something with Hammer. In the past i used to map on unreal engine. So if i just made a noob mistake, please forgive me.