The word 'taco' likely originated in the 18th-century silver mines of Mexico. At the time, 'taco' referred to the little explosives used to extract ore—pieces of paper wrapped around gunpowder. These were inserted into holes carved in the rock. The first culinary reference to a 'taco de minero' (miner's taco) appeared in the late 19th century. Similar to the miners' explosives, the food consisted of a tortilla wrapped around a filling. Tacos remained a working-class food until the mid-20th century. The 'Hard Shell' taco, popularized by Taco Bell in the 1950s, was an American innovation designed to allow for mass production and a longer shelf life, whereas traditional Mexican tacos always use soft corn or flour tortillas.