Santa Maria Antiqua is a ruined church in the Roman Forum, and is part of the Foro Romano e Palatino archaeological site which requires a ticket purchase for access. The church itself is not always open to the public, owing to ongoing excavations which started in 2004 under the aegis of the World Monuments Fund. More pictures of the church at Wikimedia Commons.  There is an English Wikipedia article with photos and plan here.
Please note that the little building with the pitched and gabled roof which features in photos is protecting the Oratorio dei Quaranta Martiri, which was a separate little church just to the north of Santa Maria Antiqua. To get to the latter, pass to the right of this building and go through an archway in a ruined wall (unless there is a barrier). The frescoes are under the modern roof straight ahead.
This was the second Christian church known to have been founded in the Roman Forum, after Santi Cosma e Damiano. It was constructed in the 6th century during the reign of Emperor Justin II (565-78) in what was once part of a monumental architectural approach to the imperial palaces on the Palatine. This complex structure, begun by the emperor Domitian, was based on an enormous ramp and the part converted into the church was a square atrium with porticoes on two opposite sides. This was at the back of the Temple of Castor and Pollux( now the famous three columns standing alone). In the mid 6th century this edifice was converted into a guard house to protect the main approach to the Palatine, still then regarded as the headquarters of the Emperor at Rome even if he was based at Constantinople. Four brick piers were inserted to hold up a roof covering the once open atrium, and the walls were decorated with Christian murals.
However, its career as a guard chamber did not last long. The building was converted into a church within a couple of decades. S. Maria Antiqua was a diaconia. It existed to provide the Church's charity to the public. The brick piers were replaced with four granite columns having capitals, the porticoes were converted into two side aisles and an apse was carved out of a solid brick wall. The interior was richly furnished, with mosaics, polychrome marbles and many frescoes. The first decoration was done during the time of Pope Martin I (649-653). The painting included the presbytery and much of the nave. During the reign of Pope John VII (705–707), the church was used by the Holy Father as his cathedral in his capacity as bishop of Rome. It was also a centre of Eastern-inspired piety in the city. John VII was responsible for the re-decoration of the presbytery and new frescoes in the Chapel of Medical Saints. During the time of Pope Zacharias (741-752), a nobleman of the same name patronized the decoration of the Chapel of Theodotus. Theodotus was the Pope's ambassador to the court of the Franks. Pope Paul I (757-767) ordered the final re-decoration of the apse and many wall paintings in the naves. The final paintings come from the time of Hadrian I (772-795).
The Oratorio dei Quaranta Martiri was a separate place of worship to the north, on the other side of the ramp leading to the Palatine. However, it was probably administered as part of the church.
The church was partially destroyed in 847, when a major earthquake undermined the imperial buildings overlooking the Forum, and the church was buried under rubble. This is now thought to be the event that destroyed the Forum as a functioning civic centre. A new church, known as Santa Maria Nuova (now Santa Francesca Romana) was built near the Arch of Titus as a replacement. The old church is thought to have been completely buried in 1084, when nearby buildings collapsed during the fire started by the Normans who sacked Rome that year. However, this is a surmise and archaeological evidence is wanting.
A new church, Santa Maria Liberatrice al Foro Romano , was built on the site in the 12th century, on top of the ruins of S. Maria Antiqua. In 1702, the apse of S. Maria Antiqua was accidentally rediscovered and partially excavated, revealing a figure of Paul I. However, soon after this discovery the excavation was filled back in. It would not be reopened until 1900 when S. Maria Liberatrice was demolished to make way for excavation of S. Maria Antiqua. The project was lead by Giacomo Boni. During this excavation, many of the frescoes were exposed to the weather, leading to their further deterioration. In 1910 a roof was constructed over the central nave to protect it from deteriorating. Many restoration efforts have been begun since 1900, culminating in the creation of the S. Maria Antiqua Project funded by the World Monuments Fund with the goal of preserving a documenting the church for generations to come. Eventually, the church is expected to become open to the public.
There are a number of frescoes, some of which are well preserved, dating from the 7th and 8th centuries. Pope Martin I (649–655) had the decisions of the Lateran Council of 649 recorded on the walls, principally rejecting the Imperial theology of Monotheletism . For this he was exiled and martyred.
Some of the frescoes are known to have been ordered by Pope John VII, and other by Pope St Zacharias (741–752). At the end of the left aisle, in the Chapel of Theodotus, there is a Crucifixion scene, dated between 741 and 752. Christ is dressed in a blue garment with Roman bands of rank. He is flanked by the Blessed Virgin and St John the Evangelist, and the smaller figure is Longinus, the Roman soldier who pierced Christ's side. Below is a portrait of Pope Zacharias, the donor Theodotus, an official of the diaconia, the martyrs Quricus and Julitta and the apostles Peter and Paul flanking the Virgin and Child. The other paintings are damaged, but one can still make out some details. The painting of St Anne holding the Blessed Virgin in her arms, dated to 649, can be identified from a Greek inscription, as can the painting of the Maccabees (also 649). In the same aisle there is a Christian sarcophagus with a relief of the story of Jonas, which is symbolic of resurrection.
These frescoes, which were often painted on top of the older works, create a palimpsest and document the changing styles in medieval Roman art. There is a heavy Byzantine influence not only in the artwork but also in the materials themselves that were used: "Many of the painted plasters are characterized by a high binder (lime) ratio and by the presence of vegetable fibers (wheat straw or husk) in the mix. This particular composition, rather unusual for Rome, confirms the Byzantine tradition or even the Eastern provenance of several workshops involved in the decoration of the church" (Soprintendenza Speciale per i Beni Archeologici di Roma). This is one of the reasons S. Maria Antiqua fascinates scholars.
Note on Temple of Castor and PolluxEdit
The famous set of three isolated columns, supporting a fragment of entablature, are just to the north and have featured in depictions of the Forum since mediaeval times. The interesting thing about them is that the rest of the temple was demolished to its foundations, so how did this threesome survive? One possibility is that the earthquake in 847 caused a lateral seismic wave which pulled down all the columns on one side, dragging the roof off instead of collapsing it and hence leaving the three columns intact on the other side. However, a recent alternative theory is that they were part of a church in early medieval times, and hence were preserved from ruination. Documentation seems lacking.