[radish]

This is my first post, so I'm sorry if this has been covered before, but I haven't been able to find any information on this anywhere else on line.

I just moved into an old Victorian apartment. I want to hang some pictures and put some shelving units up on the walls. Every time I drive a nail into the wall it goes about halfway through, then stops. It hits something really spongy and weird on the other side, and some soot colored dust comes sprinkling out of the hole. I can't get the nail to go through straight since this material isn't firm.

Does anyone have any experience with this? I'm not really sure how I should deal with this stuff, since I'm renting I don't want to do anything that's going to make gigantic holes. Thanks a lot!