User blog comment:IamJakuhoRaikoben/Chapter 372: Breach, Review./@comment-4449950-20140312110503/@comment-7418318-20140312152158

I think the poison part is also a good explenation why nobody found it.

The only ones who'd get close to the poinsonous area would be ones who harvest the jelly or plants (or whatever it will turn out to be) for some reason and they'd stick to the edge of the area, rather than going into the middle.

It also makes Wendy a prime candidate to go there, since she can heal poisoning in herself, as she demonstrated against the Garou Knights. The others would be Mira ("Poison is a treat to a Demon") and Cobra.