Its discovery dates back to the first half of the 19th century, when Domenico Rossetti, in 1825, designed the mosaics of the apse of the church which was brought to light only at the beginning of the 60s of the last century. In 1963 some workers of the Municipality of Trieste, while carrying out ordinary works, found, at a depth of two meters from the ground, a polychromatic mosaic fragment, with the inscription Bonosus defensor Sanctae Ecclesiae Tergestinae.
Under the direction of the Superintendency of Monuments, Galleries and Antiquities of Trieste, archaeological excavations began which brought to light an early Christian basilica.
The Basilica was located outside the Roman and then medieval walls of the city, in the cemetery area, upstream of the commercial road which, following the seashore, served the Roman port and continued towards the necropolis between tombs and funerary buildings. The Basilica was probably born to house the relics of San Giusto, whose body was found on the seashore. No news of the church was found until 1150 when it reappears with the title a Sancta Maria ad Mare. In 1655 the church was completely destroyed by a violent fire and only a few fragments of a mosaic were saved from the flames. It was rebuilt in 1658 and following the ecclesiastical reforms wanted in 1784 by Joseph II, son of Maria Theresa of Austria, the Confraternity was dissolved and the Church was sold to the shopkeeper Bernardo Curti, who demolished it to make way for a house. .
Scholars have identified two construction phases of the building corresponding to two floors thrown only 5 centimeters from each other in the hall of the faithful.
The date of the first phase is set between the end of the fourth and the beginning of the fifth century, while the second is between the end of the fifth and sixth centuries. The oldest part belonged to a basilica with a single nave 30 meters long and 11 wide, without an apse and dating back to the 5th century, the second had a cross plan with the erection of two bodies on the side of the presbytery. The basilica, oriented to the east, consists of a cruciform hall with a semicircular transept and apse inside and a pentagonal outside. To the south, the presbytery communicates with a rectangular room and it is conceivable that at the time another room also opened to the north (pastiphoria). In the slightly raised presbytery, two sarcophagi were found, buried and uncovered, and a well for relics that was to form the center of the altar, of which a large Istrian stone is preserved a little further south.
The oldest mosaics, with gray and white tiles, are divided into three parallel lanes and have geometric motifs, while the more recent and polychrome upper ones have three parallel lanes, bordered by a braid with two ribbons. The central lane, although it is only partially preserved, is decorated with the typical marine wave motif. The northern lane is made up of the intertwining, in two rows side by side, of two large diametrically opposed rhombuses, enclosing an octagon in the center, more often containing an inscription.
The inscriptions inserted in the mosaic floor contain for the first time the appointment to the Sancta Ecclesia Tergestina and the names and social status of those who incurred the construction costs, providing a precious document relating to the wealth of the Trieste Church. Interesting are the names of the Defensores ecclesiae lay officials who were entrusted with the legal protection of the churches of Aquileia and Tergeste in civil and administrative disputes and among the various donors there are also many names of Greek and Eastern origin, testimony of the remarkable relationships maintained by the city with those regions.