The entrance to the second layer was a huge cave with a diameter of a hundred meters. As they approached, they could see why it appeared so dark when viewed from above.
Within this vast cave, a dense black ash dust filled the air, putting one's eyesight to the test. A mere ten meters of visibility was considered quite good.
Through Soul Communication with the Fire Crow Commander, Ji Dong learned the secret of this passageway. It turned out that each of the eighteen layers of the Inner Earth World had its own prohibitive features. For the transition from the first to the second layer, the restriction was this toxic black ash dust. The dust was poisonous but not extremely potent. It was bearable for the Fire Crow Clan, but lethal for the more fragile Fire Essence Race. This was one of the reasons why the Fire Essence Race only dwelt on the first layer of the Inner Earth World.