I'm still convinced there's a door beside the mural atop the mountain. It clearly looks like one and there's plenty of space around it for there to be a room behind it. I believe it's an elevator going down into the mountain, that's what the mural looks like to me, a straight shaft with multiple rooms. I'm not 100% yet either but I've been reading about this mystery for days.
I've been there when the UFO is around and when it isn't. I've been there with and without the Space Docker and I've even tried to explosives and crashing into the door with the Space Docker; yes, I've tried the horn.
I doubt there's an elevator or secret tunnel if Mt Chilliad was once a volcano.