This is St. Paul's cathedral. The cathedral was designed by Christopher Wren to be re-built after the Great Fire of London in 1666. Even if you are not particularly religious, you can still go and view the beauty of the cathedral.
30 famous places that you MUST see. The Patriarchal Cathedral Basilica of Saint Mark is the cathedral church of the Roman Catholic Archdiocese of Venice, northern Italy. It is the most famous of the city's churches and one of the best known examples of Byzantine architecture. Wikipedia