There's one BIG 3 tile marvel on the map, and several small 1 tile marvels on the map. In order to trigger the quest, any player has to find the big one. In order to get a bonus, you then have to find 3 of the small ones AFTER the big one was found, send a unit on it, and you'll have a button to explore it. Any unit that can go on the tile can do this.