(Luke 3:19-20; 7:19-23)
John the Baptist was put in prison by Herod the tetrarch (also called Herod Antipas) because John rebuked Herod for his evil deeds--especially for taking Herodias as his wife. Herodias was Herod's niece and also his brother's wife.

Even though Herod Antipas was married to the daughter of an Arabian king of Petraea, he was enamored of Herodias, the wife of his half-brother, Philip I. During a visit to Philip's home, Antipas and Herodias eloped.

Herodias had a daughter by Philip and thus, by marrying Antipas completely disregarded Jewish law. This was especially distasteful to the Jews because she was a Jewess and belonged to the royal family.
( Merrill F. Unger, Unger's Bible Dictionary, Moody Press, Chicago, Illinois, 1974, 472.)

Archaeology

In ancient times, the place of the prison where John the Baptist was held was called Machaerus. It was located on the eastern shore of the Dead Sea, between Abarim and Pisgah. It was a rock-hewn dungeon thought to be beneath the splendid banquet hall in which the nobility were entertained, today it is called Mkaur.
(Merrill F. Unger, Unger's Bible Dictionary, Moody Press, Chicago, Illinois, 1974, 473.)