The Amber Room
In the over-the-top opulent Catherine Palace near St. Petersburg, Russia, there was a room that was adorned in gold and jewels, with huge wall panels made from hundreds of pounds of amber. When German troops captured the palace in 1941, the panels and gilded decorations were removed and sent to Germany.
They disappeared from history after that. In fact, there is no evidence the amber panels even made it to Germany. They could still be in Russia, or anywhere else for that matter.