The cavern is located under the groundwater in the glacial moraine. One can have a look at the rock through the window drilled in the wall of the cavern just on the right of the door when entering the cavern. Water is dropping there. Water is elevated to the surface by means of a set of pumps. There is no danger of radiations whatsoever even during the LEP operations, since the detector and the concrete shieldings are sufficient to stop the synchrotron radiation which would be the main cause of radiations.
Collisions are negligible in that respect.
Stay on the bridges, no trespassing onto the detector. Mind your heads when on the passerelles on the sides of the cavern. During shutdowns, watch the endcaps, then climb onto the side bridge of the cavern from which you have a nice view. Watch the endcaps, close to the entrance. They are about 10m high and weight 600 tons each.