Dante's Gate

Dante's Gate is located near the Hidden Star Village. Sumaru discovered it after he exorcised the first Basara. When he entered, he found that he discovered a place that existed where it shouldn't exist. Sumaru immediately turned around and ran through the gate which was closing. He leapt out at the last second and the gates sealed shut. He sent a secret missive to Ryo with notes on this place.

Experimentation
Sumaru began experimenting with the gate. He took notes on it's shape, size, age, etc. He took his notes and measurements back to the lab where he and his wife, Hokuto, began comparing notes. They made the discovery that the gate is actually older than the village itself. It has apparently been around since Kaguya's time. From very early manuscripts, it stated that the gate opened once every 50 years and that anyone who had entered had never come back. Due to this, research about it has been stopped since Second Shinobi World War. Sumaru then bore witness to what he saw in there in the five seconds he went through the gate.

Library of Decease
The Library of Decease was what the place that Sumaru saw was called. In another ancient manuscript, it was written that someone managed to get a note of what they saw through the gate before they died. The writers who saw the note named the place the Library of Decease. More Info Soon