On reaching the limits. The early explorers said the North had a void at the center that was not always findable. At the south a 100 foot wall all the way around, a short plateau, then a 10,000 foot tall ridge, this extended far out of sight, it had a few features like mountains and rocky outcropping. It was said to be rich in minerals, coal uranium. It would be a huge task to explore past this point, but I think they did. What they found is not public. The restricting of access was put in place at this time. A bit of a mystery.