The traditional stone-lined oven used to cook agave for tequila is called a horno.
A horno is commonly an underground or masonry oven in which agave piñas are heated for an extended period. Cooking converts complex carbohydrates in the plant into fermentable sugars, while also softening the piñas for extraction.
Modern tequila producers often use steam-pressure autoclaves or continuous diffusers instead. Those technologies can be faster and more controllable, but the horno remains important in traditional production and in descriptions of historically rooted tequila methods.
“Horno” is sometimes mixed up with “tahona.” A horno cooks the agave; a tahona is a heavy stone wheel or the milling system used later to crush cooked agave fibers.