Two or possibly more sub-structures inside of the Ancient City contain sus gravel, which always have nothing in them.

There may be more, but ill provide images for proof of two structures i found that seem to always have 0-item sus gravels. To cover things, Im sure nothing was in them, i showed preston the mod as well of one of these cases ( sorry about the warden wasnt my intention to bring you into that chaos )- and im sure it wasnt a disk five fragment, those things are tiny but dusting the gravel didnt spit those out.

The two structures that i found had a zero item gravel were the mini portal mouth structure, and the pillar tower structure ( I dont know their official names sorry ).
Ill slide in some images too for proof and context of both sub-structures:

Image one is the pillar tower ( two shriekers on the top and bottom of the structure, and two chests- one on top and one on bottom.

Image two is the portal mouth structure

thankfully both of these are just 1 broken sus gravel, but its still good to report. Not the end of the world that i found a few duds or that others have, but still an urksome thing XD.