What is the structure that Luke enters while training with Yopa on Degobah during The Empire Strikes Back?
It wasn't until I read this awesome article about Ralph McQuarrie that it dawned on me that yes, what is that place that Luke ventures into on Dagobah when is is undergoing training with Master Yoda? It seems to be a cave but it has like ummm structure.
Ross Plessat asked McQuarrie and wrote:
"Hoping to learn about the stone hallway on Dagobah where Luke confronts “Vader,” I pointed to his sketches of it and asked, “Is that a remnant of a mysterious civilization?” “It must be,” he answered. “Somebody lives there, Yoda lives there. I don’t know very much about Dagobah.“ I replied, “In an early script Yoda tells Luke that Dagobah had been used as a training ground for Jedi in the past, so maybe this structure was part of the academy.” “Could be,” he said. “There must be some barracks or something left there. [Laughs] I don’t know; I didn’t get into it with George.”
Training ground in the past? Seems like not too ban an idea. But why would Yoda hide so close to such a place? Couldn't Vader find him? Maybe it was a place so ancient that only people who are 900 odd years old would remember such a thing.
Perhaps the Extended Star Wars Universe can help
The cave / structure itself is sometimes referred to the Dark Side cave. This is possibly because Dark Jedi practiced there and maybe died there.
But frankly unless George Lucas spells it out in terms of the movie, nothing will explain it.