Mounds of Krakus and Wanda: Earthen Mausolea of Legendary Polish Aristocracy?
On a visit to Krakow there are some remarkable earthworks to behold. Of the four that exist, two are fairly modern, being built in the 19th and 20th centuries. The other two have much more age to them, but as yet there is no agreement as to what this might be. Arguments range from 500 BC to the Middle Ages.
Source: ancient-origins.net