OK, for scale, the round things on the right side are about 4" in diameter.
Geez, way to go PontiacDad, close enough to be correct!
It is actually a shot of the rear tubesheet, firetubes (to the right), flexible staybolts (left) and crownsheet (on the bottom of the staybolts) on the waterside of a 2250 horsepower steam locomotive boiler. Took the picture this morning after cleaning for required annual maintenance.
I really thought that would stump y'all longer than the first guess!