sure, you got info back. I just thought that was the most relevant and immediate piece of info. I didn't want to dilute the impact.
Back at base, after your report, as you are getting pulled apart, cleaned, greased, adjusted, and reassembled, you sift through your findings from that event. Whatever it was that looked back through you was a large consciousness, but perhaps not a sapient one. it lived, not in the sand, but in the underground regions of the ruined city. The entire city, above ground and below, carried with it the feel of age - many years in a dry place above, many years in the cool earth below, and deeper, the typical scent of dampness. The below ground part of the city was layered - the higher layers looked as if they were meant to be lived in above ground, but perhaps had been consumed by the desert. Below that, a mix of the remains of previous cities, long since built over, and a vast, intricate tunnel network. Much of the tunnel network was designed as habitation and commerce, while still more was designed as waste removal, maintenance, drainage, and the like. Even further in, natural caves could be found.
It is unclear whence came the consciousness that resides there now, or if it is one vast gestalt, or if you brushed up against a sort of collective unconscious from some kind of hive or nest or something. But other beings live within the ecosystem of the underground city as well, though you only caught glimpses of the usual small things: insects, rodents, lizards.
You send this info as a follow up report. Next time you approach the portal room, that portal is shut, and tagged with some arcane writing.