The Guanajuato mummies were discovered in a cemetery of a city named Guanajuato northwest of Mexico City. They are accidental modern mummies and were literally "dug up" between the years 1896 and 1958 when a local law required relatives to pay a kind of grave tax. You could pay the tax once (170 pesos) and be done with it; this option may have appealed to wealthier individuals. But you were also allowed to pay a yearly fee (20 pesos); this would have appealed to less wealthy families. However, if the relatives could not pay this yearly tax for three years, the body (which had, by the way, become accidentally mummified) was dug up from the cemetery and (if the fee still wasn't paid) placed on display in El museo de las momias. [Of course, what if the person's family had moved from town--or what if the person was the last person from their family? Well, it didn't matter; the law was the law!]
Fortunately, in 1958, the law was changed. Although no new bodies have been exhumed, the museum still displays the original mummies.
A combination of the soil conditions and a dry climate of the mountainous area caused the bodies in the local cemetery to dry out naturally before they could decompose. The bodies were placed in tombs, seven rows high; apparently, those in the middle row were more likely to have been naturally mummified. However, only those who failed to pay the grave tax were ever exhumed so information about mummification will always be incomplete.