Baths of Trajan

Start with the ground under your feet. Here on the Oppian Hill, the gravel paths crunch softly, and umbrella pines throw patches of shade across the grass. Between the trees, huge walls of thin red brick rise out of the lawns. They look like broken pieces of a cliff. They are what remains of one of the largest public bath complexes ever built in ancient Rome.

These are the Baths of Trajan. The emperor Trajan dedicated them in the year 109, early in the second century. Trajan ruled at a moment of great confidence for Rome, when the empire reached close to its widest extent. Building projects on this scale were political statements. They told ordinary citizens that the emperor shared the wealth of conquest with them, in the form of hot water, marble and leisure.

The design is usually credited to Apollodorus of Damascus, the architect and engineer most closely linked with Trajan's reign. He is also associated with Trajan's Forum and its great market, not far from here. Whether he drew every line of this complex is not certain. But the bold, rational plan fits his reputation well, and many scholars accept the attribution.

Now for the first surprise. Under your feet, buried inside this hill, lies something older and stranger. After the great fire of 64, the emperor Nero built a vast palace here called the Domus Aurea, the Golden House. It was a pleasure palace of painted halls, gardens and an artificial lake. After Nero's death, his successors wanted to erase his memory. Trajan's builders filled many of the palace rooms with earth and rubble, and used them as foundations for the baths.

Think about what that means. The private luxury of one hated emperor became the hidden base for a public gift to the whole city. The message was clear, and very Roman. Nero took land for himself. The new emperors gave it back to the people. The Colosseum, just down the slope, was built on the site of Nero's lake for the same reason.

The baths themselves followed a plan that later emperors would copy for centuries. A huge rectangular enclosure surrounded open gardens and walkways. In the centre stood the main bathing block. Visitors moved through a sequence of rooms: a cold room called the frigidarium, a warm room called the tepidarium, and a hot room called the caldarium. There was also an open-air swimming pool, known as the natatio.

Here is a detail most visitors miss. The complex was turned towards the southwest. This was deliberate. The hot rooms faced the afternoon sun, which is when Romans usually bathed, after the morning's work. Large windows caught the light and the warmth. The architects were using the sun as part of the heating system, alongside the furnaces and the hollow floors that carried hot air underneath.

A Roman bath was much more than a place to wash. It was a social club, a gym and a cultural centre in one. People exercised, gossiped, made business deals and ate snacks sold by vendors. Bath complexes of this size often included spaces for reading and lectures. Rich and poor could share the same halls, although the rich arrived with slaves to carry towels and oils.

Now look closely at the surviving walls. You will see the typical Roman building method of the time: a core of concrete, faced with neat rows of flat bricks. Originally, almost none of this brick was visible. It was covered with marble panels, plaster and paint. The curving wall sections you can see are parts of exedrae, large semicircular halls set into the outer enclosure. Some still hold the shape of their half-domes.

The second surprise sits a short walk away, at the northeastern edge of the park. This is a massive underground cistern known as the Sette Sale, which means the Seven Halls. The name is a little misleading, because the structure actually has more than seven chambers. It stored the enormous quantity of water the baths needed. Its long, parallel vaulted rooms show how much careful engineering stood behind every hot pool.

The baths stayed in use for several centuries. As the western empire declined, the aqueducts that fed the city were damaged and neglected. Without a steady water supply, the great complexes of Rome slowly fell silent. Over time, people took away the marble and metal fittings for new buildings. The brick and concrete cores were too heavy to move, so they stayed, and the hill gradually grew over them.

The buried palace below had its own second life. In the Renaissance, young artists lowered themselves on ropes into the dark rooms of Nero's palace, which they thought were caves. They copied the painted decorations by torchlight. Because the rooms seemed like grottoes, the style became known as grotesque. So a word still used today in many languages grew from the darkness beneath these baths.

Today the area is a public park, the Parco del Colle Oppio. Locals walk dogs here, children kick footballs against ancient walls, and older residents sit on the benches in the shade. It is an ordinary city park built on top of an extraordinary layering of history. That mix of everyday life and deep time is something Rome does better than almost anywhere.

Before you leave, stand still for a moment beside one of the great brick curves. Look up at where the vault once closed overhead. Imagine the steam, the echo of voices and the light pouring through tall windows in the afternoon. Then look down the slope towards the Colosseum. As you walk away, listen to the gravel under your shoes, and remember the painted rooms still hidden somewhere below.