http://www.guinnessworldrecords.com/explore-records

I reckon it might be this one: http://www.guinnessworldrecords.com/world-records/longest-time-survived-trapped-underground

Depends how you define "trapped underground" I suppose, but I reckon I could spend a month and a bit in a basement if I had stuff to entertain me, grub and drinks. Might be quite nice actually.