Sooo... I think I finally managed to pin down the inheritance stuff of templates and entities. I no longer need to make a difference in the save game bewteen the underground and the entities that are placed on the underground (which is a good thing because it makes my live so much easier) and the loading/saving of all that stuff is much easier than I thought.
On another topic : Guild Wars has it's 3 days open beta. Today is day 2 and I'm playing it. It's really cool. I've participated in the E3 Preview event and had a chars back then which is still accessible now... Nice move... They managed to enhance the whole game without changing the player data.
If anyone reading this is playing guild wars, look after Mirau Duval and invite him into your party ;)