They “discovered” it the same way europeans “discovered” America. It’s 15 minutes from a paved road. Even if it was two days into the jungle there’s no way local people didn’t know about this.
Chicleros from the Yucatan peninsula know the jungle very well. They stay in the jungle for months at times, national borders practically don’t exist and there’s some path to get anywhere if you’re willing to hike multiple days and manage to get water and food, which they do.