The fifth layer did not look like an underground bunker and instead it looked very much like a normal city but just without the sun. It was also a lot less dilapidated than the city which was in the outside world.
Most of the explorers who had come to these ruins were unable to enter the fifth layer and ended their journey on the fourth. That was because there was a mechanical guardian protecting the doors to the fifth layer. That mechanical guardian was made in the shape of a Griffin and it was incredibly strong.
If Elena judged that mechanical guardian according to levels then that guardian's level would be 250 plus. So it was almost impossible to get rid of it without deploying extra large weapons.
But after a lot of experiments and sacrifice the explorers had found a way to distract the mecha Griffin and used that chance to enter the fifth layer. But the cost of items they needed to distract it was very high so not many could use this method.
