There are at least 5 or 6 sub-levels of substantial area each which would have to be supplied and maintained. From the drone video you can see one or two other garage like, lower openings. Plus, there was likely a lot of use of the underwater submarine supply. I don't think we ever ruled out that there might be a tunnel to another island. So, that could very well be a staircase which would be in keeping with the Temple design.