|Located in||Valley of Broken Spirits|
|Affiliation||Hattori Clan, Masanari Black Flowers, Archaeopteryxs|
The Hollow Mountain is a large hollowed out mountain located within the Blue Marble Forest of the Valley of Broken Spirits on Aileron that houses the secret location of Ueno Castle, home of the Hattori Clan and the Masanari Black Flowers. Because of the hostile wildlife in the Valley of Broken Spirits, humans rarely ever travel to this mountain.
The hollow mountain is a large mountain located within the Blue Marble Forest. As the name suggests, the mountain is hollowed out and contains Ueno Castle inside it. The entrance to the inside of the mountain is located at Ankyla Falls, which sits at the base of the mountain. The top of the mountain is actually open, allowing MBF agents to leave through it for missions, but due to special and powerful enchantments, it appears as a regular mountain peak from the outside. A thick mist covers the entire mountain, making it completely hidden from view while also making it difficult for any aircrafts to travel over it. This is further exacerbated by the hostile wildlife that inhabit the Valley, many of which have been known to attack with the slightest provocation. MBF crafts are unaffected as, even though they're equipment is affected as well, they have prior knowledge of the mist and can navigate through it safely.
When the Hattori Clan was cast out by the Tokugawa Clan, Hanzo Hattori selected the hollow mountain as the place to move his clan to safety. Ueno Castle was constructed and the Hattori Clan has lived there peacefully and safely since. The location is also used as the base of operations for the Hattori Clan's mercenary organization, the Masanari Black Flowers.
The Hattori Clan shares their home with the archaeopteryxs, a species of reptilian, dinosaur-like birds. They peacefully coexist with the Hattori Clan and many have been used as mounts to get around.