Discover Santa Maria degli Angeli e dei Martiri

Santa Maria degli Angeli e dei Martiri is a basilica built inside the ruined frigidarium and tepidarium of the Baths of Diocletian, on Piazza della Repubblica. Turismo Roma records that until the mid-sixteenth century the baths appeared only as a grand ancient legacy, and that in 1561 Pope Pius IV charged Michelangelo, then eighty-six, with transforming the complex into a church; the reference work dates his work to 1563-1564. The impetus came from a Sicilian monk, Antonio del Duca, who had lobbied for decades for papal authorisation.

Michelangelo integrated the church into the baths while keeping the original Roman structure of over ninety metres, carried on monoliths of granite from Egypt. The front is not a facade at all: its singular concave form in brick is one of the ancient exedras of the calidarium. After Michelangelo's death in 1564, Jacopo Del Duca continued the work.

Luigi Vanvitelli directed later construction — Turismo Roma says he designed a new facade in 1750, the reference work puts his work at 1749 — and in 1911 the Vanvitellian front on Piazza Esedra was demolished to restore the calidarium niche in Roman brick.

The meridian line, inaugurated in 1702 by Pope Clement XI and built by the astronomer Francesco Bianchini, runs forty-five metres in bronze enclosed in yellow-white marble, tracing the sun through the signs of the zodiac. It was made to check the accuracy of the Gregorian calendar reform and to predict Easter exactly.

Open every day, 08.00-20.00. No admission is charged.

The architecture is Roman and the architect's job was to leave it alone. Michelangelo adapted a section of the remaining structure to enclose a church rather than building one, keeping — in Turismo Roma's words — the original Roman structure of over ninety metres. The vaults overhead are the bath's; the monoliths of granite from Egypt that carry them are the bath's. Almost nothing structural is sixteenth-century.

That decision produced the building's one great oddity. A bath hall runs crosswise, so the church is entered on its long side: you arrive in the middle of an enormous transverse space rather than at the end of a nave, and the altar is off to one side. Every expectation a visitor brings about how a church is organised is wrong here, and it is wrong because the room is older than the expectation.

The front is a ruin, deliberately. Turismo Roma describes the facade's singular concave form in brick as one of the ancient exedras of the calidarium. There was an eighteenth-century front by Vanvitelli, and in 1911 it was demolished to put the Roman brick back on show — a reversal that is itself a statement about what the building is for.

And there is an instrument built into the fabric. The meridian line of 1702 is forty-five metres of bronze enclosed in yellow-white marble, laid across the floor by Francesco Bianchini, with a hole admitting sunlight from high in the wall. It makes the church a piece of scientific apparatus as well as a place of worship.

It was not built. It was converted, by subtraction, at the end of one man's life.

The raw material was already there: the frigidarium and tepidarium of the Baths of Diocletian, still standing and still roofed when every other imperial bath in Rome had gone. Turismo Roma puts the commission at 1561, from Pius IV to an eighty-six-year-old Michelangelo; the reference work has him working in 1563-1564. He died in 1564, and Jacopo Del Duca carried it on.

The work was about what to keep. The granite monoliths from Egypt that held up the vaulted ceilings and arches stayed; so did the ninety-metre hall and its cross-vaults. What changed was use, and a floor level.

The eighteenth century added and the twentieth took away. Vanvitelli directed later construction and put a facade on Piazza Esedra; in 1911 that facade was demolished to restore the suggestive niche of the calidarium with Roman bricks. The building has been edited in both directions, and the editors of 1911 preferred the ruin.

The most precise piece of construction here is the newest of the old. In 1702, for Clement XI, the astronomer Francesco Bianchini laid a 45-metre bronze meridian into the pavement — work that required the building to be surveyed, a true north established and an aperture cut, to produce a tool for checking the Gregorian reform and predicting Easter exactly.

When is it open? Every day, 08.00-20.00, weekdays and holidays alike.

Does it cost anything? No. Neither the basilica nor Turismo Roma publishes a price.

Is this really inside the Baths of Diocletian? Yes — it occupies the ruined frigidarium and tepidarium, and Turismo Roma records that Michelangelo kept the original Roman structure of over ninety metres, carried on Egyptian granite monoliths.

What did Michelangelo actually do? He adapted a section of the remaining structure to enclose a church. Turismo Roma says Pius IV commissioned him in 1561, aged eighty-six; the reference work dates the work to 1563-1564. He died in 1564 and Jacopo Del Duca continued.

What is the brass line in the floor? A meridian of 1702, forty-five metres of bronze in yellow-white marble, built by Francesco Bianchini for Clement XI to check the accuracy of the Gregorian calendar reform and to predict Easter exactly.

Why is there no facade? There is — it is one of the ancient exedras of the calidarium, in bare brick. Vanvitelli's eighteenth-century front was demolished in 1911 to uncover it.

When is Mass? The parish gives weekdays and Saturday at 18.30, and Sundays at 09.00 (English), 10.30, 12.00, 17.00 (Spanish) and 18.30. Turismo Roma lists the weekday Mass at 18.00 instead.

How long does it take? About 40 minutes.