At the visitors center I got the necessary information and booked a 2pm guided tour to Kings Palace. During the previous two hours I toured the Big Room. I went down the 750' by elevator (1 min). Down there, i realized, that there is no chance to get reasonable photos without the tripod, so I went back up and picked up the tripod as well as a light jacket. The temperature in the caverns is a constant 56F with high humidity.
I returned from the big room just in time to go on the guided Kings Palace Tour. About 50 people, with all kind of cameras were on this tour. The four guys with tripods had been asked to form the tail of the tour to not obstruct the others with our tripods, which was fine.
Both tours were very interesting. Interested readers find a lot of technical information about the geology and history of the caverns here. Taking pictures was not easy, since there was very little light and one could not bring any light and flash lights are definitely not the right thing to do.
Whatever I got out of my equipment can be seen here.