In the shadow of the American petrochemical industry, hundreds of workers perform a crucial, dangerous and utterly invisible task. They physically climb inside tanker trucks and rail tank cars and chemical barges to clean them out. The men, nearly all black and Latino, come in direct contact with fiercely corrosive chemical leftovers and fumes, often working with scrapers and razor blades to get every square inch of the insides clean. Some say they do so without so much as a mask. Many have died. Generating data from scratch, frequenting truck stops, knocking on doors and collecting obscure business sources, the Houston Chronicle provides a first instance look at this previously unknown industry, cross referencing every found workplace with OSHA records to paint a picture of a labor that has passed mostly under the radar.