Okay this is the dungeon in worstseedever.
Looks man-made, right? It's not.
Under sand block 1, is the chest.
Under sand block 2, is the mob spawner.
Coordinates are approx.
x - -2
y - 65
z - 7
Edit: Okay, the moss stone is under the sand, but under the moss stone, is a cave.
Guess what's in this cave? A dungeon, with a large chest, not a single chest.
This is a cool seed :D
So, once you find the cave under the first dungeon, you should land on/near some gravel. The cave goes two ways, but one way is just a dead end, so go both ways,
One way, there's just stone, and some iron ore in the corner.
The other way, you should find some cobblestone, not stone, cobblestone.
Mine into the cobblestone and you should find the dungeon!