It wasn't something that took Henry a long time to be able to locate the areas where the other people had been sealed. Since he already possessed experience from one of the dungeons, Henry was able to open the coffins that were used to seal those people, releasing them back into the world.
Due to his teleportation skill, he was able to move around at an incredible speed. And for that reason, in just a matter of about 3 days, he had already visited all the dungeons all over the world. After releasing them, he came to realize that the reason why there were many dungeons within Akila was simply because it was the capital for the empire located on the Blazing continent.
This was the headquarters of the humans, and of course, it possessed many more strong people. And for that reason, they had been able to seal away many people, leading to the formation of the dungeon in a method that Henry did not understand.
