What is the burden of proving a crime committed without trial?

"In every criminal trial, the prosecution must prove the corpus delicti, or the body of the crime itselfi.e., the fact of injury, loss, or harm, and the existence of a criminal agency as its cause. In California, it has traditionally been held, the prosecution cannot satisfy this burden by relying exclusively upon the extrajudicial statements, confessions, or admissions of the defendant. [Citations.] Though mandated by no statute, and never deemed a constitutional guaranty, the rule requiring some independent proof of the corpus delicti has roots in the common law. [Citation.] California decisions have applied it at least since the 1860's . . . . [] . . . This rule is intended to ensure that one will not be falsely convicted, by his or her untested words alone, of a crime that never happened. [Citations.]" (People v. Alvarez (2002) 27 Cal.4th 1161, 1168-1169, fn. omitted.)