San Marino is a country in the Apennine Mountains. It is a landlocked enclave, absolutely surrounded by Italy. San Marino is considered the oldest constitutional republic in the world: it was founded on 3 September 301 by Marinus of Rab, a Christian bricklayer fleeing the religious persecution of Roman Emperor Diocletian. San Marino's constitution, dating back to 1600, is the world's oldest written constitution that is still in effect. The dominant industry in San Marino is tourism. San Marino is not a member of the European Union, but the euro is used in San Marino. Tourism in San Marino contributes greately to San Marino's economy and its GDP, with more than 5 million tourists visiting this country each year.