"This ancient cave passes under the great southern mountains of Outer Terra. But, the gate may only be opened by the blue circle key." - In game description.
The angel Samantha can be found on an island on the East end of the level.
Underpass is simply a short route to the villages south of the mountains. Head South, use the blue key, and keep heading south. If for some reason you lack the blue key, use an Ethereal Potion to bypass the door. There is no need to explore the Eastern part of this small level unless you want extra fighting or to visit the angel.