From Beelitz-Heilstätten, a large abandoned hospital complex. This place was amazing, I've never experienced anything as ghost town as this one. The area was so huge that after three hours we've seen only half of the area. The oldest buildings in the area were build in 19th century, and there were sanatoriums and war hospitals. Blog called Abandoned Berlin is a great place to learn about abandoned places in or near Berlin, especially this entry helped me a lot.
We entered the last building near the dusk. Every door and window was locked, but I managed to find a way in - this suspiciously small door, well hidden and unlocked. It was so dark we used the light of cell phones. I didn't take any pictures inside the building, I just concentrated to see as much as possible. I've read that there was some kind of serial killer hiding in these ruins few decades ago. The old building was full of horrible noises, wind was blowing through the broken windows... cell phone batteries were running low, and then we didn't find the way out. Few minutes we were lost in this huge dark building, trying to locate that tiny door. Luckily we found the door before the batteries were dead.
I definitely want to go back! I think Beelitz-Heilstätten is one of the most exciting places I've ever visited.