Ancient Ruins
kodai no iseki

The Ancient Ruins is the name given by humans to a prehistoric dungeon located in the eastern part of the Helman Empire

It is famous for its absolutely immense size, deemed as the biggest dungeon in The Continent. It is so big it has a total of 1000 subsoils underground. Obviously it's plagued with monsters of all sizes and colours but it's a tempting place for adventurers since few have ventured too deep within this mysterious dungeon, leaving humanity unbeknownst to all the secrets it might hold. 

The only party to ever reach to the bottom of it are the Legendary Five, who are well-known as Legends within the continent for their feats, but this is one that went untold to the majority, since what they found at the end of their quest in the Ancient Ruins is what led to their mysterious disappearance over 1500 years ago. 

In reality, the Ancient Ruins are connected with the Gods Realm, the alternate dimension where Gods live. The dungeon has a Gate, appropriately named God's Gate, with four pedestals. With the passing of time brushwood and other vegetation filled the Gate, but it's still secured with an unbreakable seal that can only be deactivated by placing four special Golden Statues on the four pedestals. The Legendary Five are the only ones that managed to find these Godly artifacts to go through the Gate towards Gods Domains. It seems that in the past, a Proto-God (an unsuccessful attempt to create a God) was used by the Gods as a Guardian, but it was defeated by the Legendary Five and apparently wasn't replaced.

Aside from the Hell Holes in JAPAN, the Ancient Ruins and the Ragnard Dungeon are the only places in The Continent that connect with the God's Realm, although no-one knows about it.