"Dungeon, huh..."
In the first place, Deathzone was created from dungeons that didn't get cleared because, well...no one could. No tower was created, and so no esper was born in this region. With no one to kill the beasts and clear new sprouting dungeons, the miasma got thicker and became a perfect home for the creatures coming out of the dungeons.
Of course, with how thick the miasma was, even if humans used a detection machine, they wouldn't be able to sense any dungeon, since the miasma coming out of the new dungeon would be masked by the Deathzone's miasma. But that didn't mean there wouldn't be any dungeon.
"It's where all those beasts spawned from, isn't it?" Bassena shrugged. "That's why they seemed to be endless."