There are four locations marked "X", "MARKS", "THE" and "SPOT". The X, in this case, actually does mark the spot. The 12 locations used in the answer are all places on Caltech's campus, laid out according to the campus map. The X lies on top of the CENTER FOR STUDENT SERVICES (CSS). Calling in the building name will trigger the end game. The team is given a choice between the two final puzzles: Castle in the Air for the on-campus runaround, or Have Fun Storming the Castle (opt out) to end the hunt without the runaround.