GFCI tripping

[Lenny]

You say you unplugged everything. Are there any wires attached to the load side of the GFI receptacle? If so, I'd pull them off to see if the outlet still trips. It probably will, which will confirm that the receptacle is bad.

[matt4x4]

GFCI's do go bad, however, continual tripping can mean a ground fault exists, therefore, I would open it up, disconnect the load side lines as suggested above, if it still trips, it's dead, if it doesn't, there's a wiring problem load side that will need addressing....
Anyone done any digging (even minor) nearby, all it takes is some slight spade damage.