The Amber Room
The Amber Room was constructed inside of the Catherine Palace near St. Petersburg. The room contained gold mosaics and gold-trimmed mirrors and carvings, as well as over 1,000 pounds of amber. Fast-forward to 1941 and Tsarkoe Selo was arrested by the Germans.
German soldiers weren’t delicate when they disassembled and dismantled everything inside the Amber Room. They took every last bit back home to Germany, and not a single piece of it has surfaced since then. One theory is that the Germans melted the gold down. If you visit the Catherine Palace, you can see a recreation of the Amber Room.