While traveling in Italy we stopped to visit The Leaning Tower of Pisa. It had never dawned on me that the tower actually had a practical purpose. As tourists, we learned that this world famous structure is actually the bell tower for the adjoining Catholic cathedral. However, for me, the most interesting building at that site was located on the opposite side of the cathedral. It was a wedding cake looking marble building containing a very large baptistry. Indeed, the building is called "the baptistry" and was constructed solely for baptizing people into the Christian family. Measuring over 160 feet high (55 meters) and over 100 feet in diameter (35 meters) this beautiful marble structure is immense.
This then brings us to the practice of water baptism. The Scriptures clearly indicate that Jesus was following a Jewish tradition when He asked John the Baptist to baptize Him. Indeed, baptism had been a long standing practice among the Jews. Converts to Judaism were baptized to wash away the impurities of heathenism by being completely immersed in running water. Actually, baptism seems to have been practiced among many different people groups which possibly included the early Egyptians (I've seen temple carvings that appeared to be some sort of Egyptian baptism in Karnak).
The Scriptures state, "And John also was baptizing in Aenon near Salim, because there was much water there..." (John 3:23) Historically speaking, a fresco ( a painting made with water colors on wet plaster) in the Rome catacombs (ancient Christian graveyards) is the earliest know depiction of the baptism of Jesus. In the picture, as He comes up out of the water, Jesus is being given a helping hand by John the Baptist. In this second century fresco, John the Baptist has short hair and no beard, Jesus may have collar length hair and a short beard.
Baptism in the Christian church began with the practice of total immersion which followed the example set by Jesus. Until Christianity was made legal, early Christians baptized converts in streams or in private homes. However, after the fourth century legalization of Christianity, congregations began to build separate buildings expressly for baptism. This continued until the practice of total immersion faded from popular use in the Middle Ages. Examples of early total immersion type baptisteries are plentiful. The one beside The Leaning Tower of Pisa is simply one among many. Historians say there are sixty-seven (total immersion type) baptistries still in existence in Italy today that date from the fourth to the fourteenth century.
Baptisteries, such as the one at Pisa, were built separate from the church because baptisms were usually conducted only three or four times a year (Christmas, Easter, Pentecost, Epiphany). In some cases, hundreds of people were baptized by the bishop, given white robes and then invited to join the rest of the church family in the cathedral next door. However, over the years, it seems that it became easier to simply pour a little water over the candidate or even sprinkle a few drops (called affusion) upon them. A Roman Catholic, Cardinal Gibbons, stated, "For several centuries after the establishment of Christianity, baptism was usually conferred by immersion, but since the twelfth century the practice of baptizing by affusion has prevailed in the Catholic Church, as this manner is attended with less inconvenience than baptism by immersion." Faith of Our Fathers, p. 275.

For Deeper Study
Baptism Through the Centuries , p.3, Henry F. Brown.
History of the Christian Church, Philip Schaff, Volume 3, p. 558-60, "Baptisteries."
World Book Encyclopedia, Volume 2, "Baptism."

Marvin Hunt (newspaper article)