The only natural process I know of that would cause the top surface to bake
into a shell is the presence of liquid water and the heat of the sun (rather
like one would fire clay in a kiln to make pottery). If that's the case it
means the interior subsidence has been caused by ice melting underneath as a
result of interior warming over a relatively short period. If one interprets
it that way it does of course indicate the presence of water, ice beneath
the surface and of course heat acting on the surface and interior of the planet.

I am prepared to believe there was liquid water at some point. Indeed
the channels on Mars do indeed show this. There are also large regions
on Mars at constant elevation which would indicate a sea which dried
out. Yes I am prepared to accept that.

There are other gases which could do the same thing. What about an
igneous origin with gases traped in lava? What about dissolved gases
under pressure?

If it was an arificial structure where is the rest. The only person who
could have made it would be ET and ET would leave behing VN technology
to maintain it constantly.

This is the point. ET would have installed VN machines in the Egyption
desert and kept the Pyramids and the Sphinx in tip top condition.