The ruins

Two American couples, just out of college, enjoy a pleasant, lazy beach holiday together in Mexico. On an impulse, they go off with newfound friends in search of one of their group--the young German, who, in pursuit of a girl, has headed for the remote Mayan ruins, site of a fabled archeological dig. Then the searchers--moving into the wild interior--begin to suspect that there is an insidious, horrific "other" among them...--From publisher description