That´s daily business over here in Germany. Everytime we dig a bigger hole in big cities or at airports that already existed during WWII, we find unexploded bombs.
Just recently (on November 18th to be exact) there was a big bomb found at Hannover, which could not be disarmed on location due to the bad condition (britisch quality....). After 12000 people were evacuated, it was transported to a military area close to Hannover airport (about 3 miles away, 1 mile south of the approach to runway 27L) and blasted there. I was flying to HAJ that night, funny to see all the police car lights flashing blue along the road and on the boundaries of the military area. first thought of an aircraft crash when I saw it, later heard that it was just another bomb finding/disarming/blasting.
You can bet that there are several unexploded bombs within the premises of Frankfurt airport.
btw. starting threads or writing replies including keywords like "bomb" instantly puts you on some blacklist at CIA,MI6,BND etc. so expect to be watched a little more closely by our "friends".