[card_zero]

Two thousand years! I thought of the stone ships too (these are burials marked by stones laid out in the shape of a ship), but I went searching for an old example and it seemed like the oldest persuasively dated one is from around 600.

[mzi]

Tjelvar on Gotland are presumed to be from 750BCE, and I don't think that is the oldest in Scandinavia.

[card_zero]

Oh, nice. That's the one I couldn't find a Wikipedia article on, though I see now it's mentioned under Boge (with a picture). https://en.wikipedia.org/wiki/Boge,_Gotland