The area where the 'key' is indicated is a hole about eighteen inches wide and a foot deep, evidently empty. The 'bug hole' is indeed about five feet in diameter, with unnaturally smooth walls which Maximillian wouldn't think was a beetle. It appears to have been some sort of boring animal, most likely insectoid - beyond that he can't be certain. The bug hole descends about 20 feet within the 40' illumination of the torch (the last twenty feet of that illumination being very dim at best), so that the slope is quite precipitous.
Lukas would think the walls were most likely built by smaller creatures than something nine feet tall. The stones are simply too small for hands of that size. There are small bits of graffiti about, mostly the sort that would be done by children, of small humanoid creatures with stick hands and fingers. The floor is made of pressed, rotted grains, hay, straw, clay, dirt and stones that have been allowed to crumble somewhat over perhaps two or three decades, and is slightly damp.
Aside from the bug hole, there is no sign of an exit.
|cross section of bug hole|