One of the dourest, ugliest of Soviet creations, the upright H-shaped Dom Sovietov (closed to the public) has been nicknamed 'the monster' by locals. On this site stood a magnificent 1255 castle, damaged during WWII but dynamited out of existence by narrow-minded Soviet planners in 1967–68 to rid the city of a flagrant reminder of its Germanic past.
Over 10 long years this eyesore was built in the castle's place, but it has never even been used. Money ran out, and it was discovered that the land below was hollow, with a (now-flooded) four-level underground passage connecting to the cathedral.