Smoke stacks are deceptive beasts: They produce mainly steam, not smoke, and what you see from the outside is not the actual stack. I did not understand that until I had the chance to take a tiny elevator up the side. I learned that they consist of two distinct columns: a liner and a shell. The liner can be made of brick or fiberglass, and it carries gases into the atmosphere. The shell is typically concrete, shielding the liner from wind and weather. There is a small space between the two layers, with access points along the way up for monitoring and repair services. This photo was taken at 500-feet elevation, looking straight down between the liner and the shell, with two access grates clearly visible below.