Raicilla is Mezcal that has been brewed from a variety of agave known as "Mules Foot" ("Pata de Mula.") 
Production centres around Puerto Vallarta in the Mexican state of Jalisco.
"Piñas" are harvested from the agave plants (see Mezcal for more detailed information), and slow baked in brick ovens for 24 hours, then crushed, usually with mallets.
Water is added to the fibre and juice mixture in a vat, then it is allowed to ferment for 7 to 10 days, using yeasts that were naturally on the plant.
The vat is then covered, and heated. The cover turns it into a still, because it has a copper coil coming out of the top.
It is distilled for about 8 hours.
Modern producers ferment in plastic vats, then transfer it to stills.
To be legally called "Raicilla", it must be distilled twice.
15 pounds (7 kg) of pinas are required to make 1 quart (1 litre) of Raicilla. From 30 US gallons (120 litres) of fermented liquid, you get 1 quart (1 litre) of final product.
Some Raicillas are aged for a few months in oak.
Raicilla is clear, or has a very slight brownish tinge. It is slightly sweet, with a harsh flavour. It is very high in alcohol, up to close to 100%. The commercial stuff tends to be 70 to 80 proof. It can have the effects that pure alcohol of any type would on you. Self-control of body mechanisms is lost very quickly,
Raicilla is usually served chilled on ice, or with some grapefruit, lime or orange juice in it. Some also drink it straight.
Raicilla was considered moonshine, and was illegal, but local police weren't overly energetic about trying to suppress it.. It was sold in side streets in re-used pop bottles. There are several legitimate producers now, and a Raicilla festival is held every spring in Puerto Vallarta.
 "Mules Foot" agave. Aka Agave Lechuguilla, Agave inaequidens or Agave maximiliana.
Though a plant called the "San Pedro cactus" is sometimes used, and the plant does contain some mescaline (an hallucinogenic alkaloid), the mescaline remains behind in the plant fibres and in the mash; mescaline doesn't vaporize, so it doesn't get passed via distillation into the final product.
The producer John Huston and actor Richard Burton reportedly got legless drinking Raicilla in 1963 while in Mexico making the film "The Night of the Iguana."
Literature & Lore
It's a myth that Raicilla was drunk by Aztec priests. The Aztecs did not have the knowledge of distillation.
Raicilla means "little root."