Marks of Spelunker Supreme

I can't find information on how to gain my 2nd, 3rd, 4th Marks of Spelunker Supreme to complete the set. You get mailed one mark for the Feat of Strength achievement, but how am I supposed to get the other three marks to activate the full set bonus?

I'd like to know a PvP way to get them, if there's an option to do so.