The Basilica of St. John is a historic church located in Istanbul, Turkey. Originally built in the 6th century, it was dedicated to Saint John the Apostle and served as an important center for Christianity. The basilica is known for its impressive architecture, featuring a large dome and intricate mosaics. Over the centuries, the basilica has undergone various changes, including its conversion into a mosque during the Ottoman Empire. Today, it stands as a significant archaeological site, attracting visitors interested in its rich history and connection to early Christianity.