So in Destiny 1 there is a door on mars located in the rubicon wastes that has an entire unused area behind it. I have seen one other person make a reddit post about this and I have a theory for what it was meant for. A common theory is that io was meant to be an expansion to D1 Mars and its all but confirmed with dataminers finding the load zone and model for terrabase Charon I the files. I linked a video I made showcasing the area. I couldn't get in but there is definitely a way. Try it for yourself if you can.