Basilica di Santa Maria Assunta, Aquileia

Look down before you look up. Under your feet, behind the glass walkways, the floor seems to move. Waves of blue and white tesserae, the small cubes of stone and glass that make up a mosaic, roll across the nave. Fish turn in the current. Small winged boys lean out of boats with nets and lines. The light from the high windows falls on them, and for a moment the whole church feels like it is standing on the sea.

You are in Aquileia, a quiet village in the far north-east of Italy, close to the Adriatic coast. Today it holds only a few thousand people. But this was once one of the great cities of the Roman Empire. The Romans founded a colony here in 181 BCE, as a frontier post facing the Alps and the lands beyond. It grew into a major port and trading hub, linked by river to the sea, and it stayed important for centuries.

The floor you are standing over belongs to that Roman world. In 313, the Emperor Constantine and his co-ruler issued what we call the Edict of Milan, which allowed Christians to worship openly. Soon afterwards, a bishop of Aquileia named Theodore built a large complex of halls for the Christian community. The mosaic beneath this basilica was its floor. It is one of the largest early Christian mosaic floors in the Western world.

Here is the first surprise. Theodore's name is right here, set into the floor. Look for an inscription in Latin letters inside a circular frame. It names Theodore and praises him for completing this work with God's help. It is rare to find a builder from the early fourth century signing his own work so clearly. You are reading a message that has waited under this church for around seventeen hundred years.

Now follow the sea toward the altar end of the nave. Among the fish and the fishermen, a story appears. This is Jonah, the prophet from the Hebrew Bible. Sailors throw him overboard. A sea monster swallows him. Later, the creature spits him out, and he rests under a shady plant. Early Christians read this story as a sign of death and resurrection. The artists told it in a playful, almost cheerful way.

Across the rest of the floor, the panels change. You will see animals, birds, baskets of fruit, and a figure of the Good Shepherd carrying a lamb on his shoulders. There are also portraits of men and women, probably members of the community who paid for sections of the floor. Many visitors walk past these faces. Stop for one. They look back with wide, steady eyes, like people at a gathering.

The second surprise is how long all this was invisible. Over the centuries, later floors covered the mosaic. For hundreds of years, worshippers walked above it without knowing what lay below. Excavations in the early twentieth century uncovered it again. That is why the walkways exist. They let you move through the church while the ancient floor stays protected, just as it has survived so far.

The building around you is much younger than the floor. Aquileia suffered badly when Attila and his Huns sacked the city in 452. The Church here survived, though, and its bishops became powerful patriarchs, leaders who ruled a large region of churches and lands. In the eleventh century, Patriarch Poppo rebuilt the basilica in grand style, and it was consecrated in 1031. Later, after an earthquake in the fourteenth century, builders repaired it again, adding the pointed Gothic arches you see along the nave.

Look up at the ceiling. It is made of wood, shaped like the inside of a ship's hull turned upside down. This fits the mood of the sea below quite well. Then turn to the left aisle, near the entrance. There you will find a small, round stone structure. It is a medieval copy of the tomb of Christ in Jerusalem. Pilgrims who could not travel to the Holy Land used chapels like this for Easter rituals.

Beneath the high altar is the Crypt of the Frescoes. Its walls and vaults are covered with paintings from the twelfth century. They tell the story of Saint Mark and of Hermagoras, whom local tradition names as the first bishop of Aquileia. According to that tradition, Saint Mark himself brought Christianity here. Historians treat this as a legend, but it shaped the identity of Aquileia, and later of Venice, for many centuries.

There is another underground space, the Crypt of the Excavations. Here you can see more of the fourth-century floors, as well as traces of an earlier Roman house. One famous panel shows a rooster facing a tortoise. Many scholars read the rooster as a symbol of light, greeting the dawn, and the tortoise as a creature of darkness. The exact meaning is still debated, which makes it all the more fun to study it yourself.

When you step outside, the tall bell tower rises beside the church. It stands apart from the main building, as many medieval Italian towers do. Locals say that some of its stone came from the ruins of Roman Aquileia, which would not be unusual here. Builders across this region reused ancient blocks for centuries. Around the square, you can also see the low remains of earlier Christian buildings.

In 1998, UNESCO added the archaeological area of Aquileia and this patriarchal basilica to its World Heritage List. Much of the old Roman city still lies under the surrounding fields. A short walk away you can find the old river port, a row of standing columns from the forum, and a museum full of carved stone. The village is small. The city beneath it was enormous.

Before you leave, take one more look at the floor. Find a single fish, and follow it with your eyes as it swims toward the altar. Someone set each of those tiny stones by hand, kneeling on this ground, in a city that no longer exists above it. Listen to the soft steps on the glass and the quiet voices around you. Then walk out into the light of the square, and notice how flat and calm the land is now.