The Paths to the Otherworld
"The Gate within the Hill"
Faerie mounds exist in both the human world and the Faerie world at once. In our world they look like green manmade hills, "hollow hills" which sometimes open to provide passage to the Otherworld.
The Elves of legend were thought to have great halls underneath these barrows. These Elven dwellings were collectively called Underhill. Within the hollow hills great Courts of old were held with feasting and revelry.
In legends, Mortals are sometimes taken into these underground estates, especially if they were thought to be a great Artist, Poet, or Musician. There, these mortals would not age a single day.