If the likes of Sir Henry Rider Haggard and Edgar Rice Burroughs are to be believed, during Victorian times the interior of Africa was crammed with lost cities full of white folks dressed up like Romans or what-have-you. Usually there was already all kinds of trouble brewing, and they would be ripe for some blundering heroes to barge in and accidentally spark off a civil war (often by rather inappropriately falling in love with the local princess/priestess). This was in fact, how Allan Quatermain met his end.
You could barely walk ten feet into the white spaces on the map without bumping into one of these places. Might make for a fun scenario.