In the server keys of the MIT, there is several signatures from "PGP Global Directory Verification Key".
Example (from http://pgp.mit.edu/pks/lookup?op=vindex&search=0xB63024F96C73DA99):
sig exp CA57AD7C 2013-07-14 2013-07-28 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2013-10-10 2013-10-24 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2013-10-24 2013-11-07 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2014-03-04 2014-03-18 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2014-03-17 2014-03-31 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2014-05-02 2014-05-16 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2014-05-16 2014-05-30 __________ PGP Global Directory Verification Key
sig exp CA57AD7C 2014-06-25 2014-07-09 __________ PGP Global Directory Verification Key
What does it mean? And why they are all expired (if "exp" means "expire")?