What does E.E.U.U. stand for?

I was watching the World Cup on the Spanish Channel. For the U.S., they use E.E.U.U. as the abbreviation. What does it stand for?

Apparently because both Estados and Unidos are plurals, the abbreviated version has two letters for each (the site below compares it to using pp. for ‘pages’ in English):

Why is the abbreviation for Estados Unidos written EE. UU. instead of simply E.U.?