Maguuma Explorer
Silverwastes does not count. What you want to do is compare the wiki maps that show white-bordered areas zone by zone with your own map, or else get together with someone who has all the areas (I do, I could help, especially if you have voice chat) and just list them off for each zone.
This latter method is how I was clued in to the last Maguuma thing I needed, that long cave in northeast Sparkfly. It didn’t look fogged on my map, it was just a brown mountain terrain area, but it counted once I went inside!
It does not. Wiki has a list of commonly missed places. I’m on my phone which won’t let me link it, but just go to “maguuma explorer” on the gw2 wiki. Otherwise, the best way to find the missing one is to look up each zone, and see which area you are missing from the list online. It’s a bit tedious but effective.
Note that the above-described technique works for all the other regions, and helped me complete the explorer achievement for all regions.
Chances are it’s the area you go to collect a part for Mawdrey.
It’s usually either Aurora Remains in Brisban Wildlands or Deirdre’s Steppes the Jumping Puzzle in Mount Maelstrom – have you checked for those two?
It might be the second Jumping Puzzle in Mount Maelstrom as well, in the northwest corner. It’s easily missable.
I got all the Jumping Puzzle achievements though (also the one in Mount Maelstrom). Thanks for all responses, it really helped me out. Forgot I could just simply /wiki Maguuma explorer.