I'd guess those exterior walls are 6-12" thick poured concrete, similar to this[1]. You can see in the 6th picture almost all of the missing wall is still there

I'd guess those exterior walls are 6-12" thick poured concrete, similar to this[1]. You can see in the 6th picture almost all of the missing wall is still there and intact, it's just lying flat on the ground. In the 12th picture you can see how thick they are compared to car wheels nearby. [1] https://www.mppengineers.com/index_files/Page369.htm https://www.mppengineers.com/index_files/Page369.htm