Nicomedia was an ancient city located in what is now modern-day Turkey, near the eastern shore of the Marmara Sea. It served as the capital of the Bithynia region and was an important center for trade and culture during the Hellenistic and Roman periods. The city was known for its strategic location and was often a site of political significance. In the early Christian era, Nicomedia gained notoriety as the residence of the Roman Emperor Diocletian. It was here that the first major persecution of Christians occurred, leading to the martyrdom of many believers. Today, the ruins of Nicomedia can be found near the modern city of Izmit.