Ctesiphon

The Taq Kasra appears on the plain south of Baghdad the way large things sometimes do in flat landscapes: all at once, without warning, already enormous before you have time to calibrate your expectations. The arch is 37 meters high and 26 meters wide. It is the largest single-span brick vault in the world. It has stood here — unreinforced, without mortar in parts, built from thin baked bricks in the Sassanid Persian technique — since approximately the third or fourth century CE, as the iwan entrance to the Sassanid palace at Ctesiphon, the capital of empires for five centuries.

One wing of the facade has collapsed — the collapse happened in a Tigris flood in 1888 — and what remains is still more imposing than almost anything I have seen standing anywhere. The surviving half rises from the desert floor with the particular self-possession of something that knows it has outlasted the empire that built it, the empire that occupied it, the Arab conquest that ended it in 637 CE, the medieval caliphates that borrowed from its architecture, and the twentieth century that simply left it there, largely unexcavated, in agricultural land between corn fields and date palms and the slow brown Tigris.

The site has essentially no infrastructure. A small museum at the base holds some Sassanid stucco fragments and explanatory panels that were evidently installed in the 1970s and not substantially updated since. A caretaker unlocks the chain-link fence and sits in the shade of the arch — remarkable shade, given the scale — while you walk the perimeter. The interior of the arch, the iwan floor, is rough earth and rubble. The stucco that once decorated the walls — elaborate floral and geometric patterns — survives only in fragments. What the hall looked like at the height of the Sassanid empire, draped and carpeted and full of courtiers gathered around the Great King: this requires an imagination the site does not assist.

What the site does give you is the physical fact of that arch, and the fact is staggering. Stand at the base and look up through the 37-meter curve and the brick changes color in layers — lighter toward the crown where the sun rarely reaches, darker at the haunches where moisture accumulates — and the whole thing seems, improbably, to press slightly toward you, as if inviting a closer inspection. The Sassanid architects who built this had the engineering knowledge to span larger than anyone in the ancient world except the Romans. The arch stands without buttresses. Its stability is a mystery that structural engineers still argue over.

I drove out from Baghdad on a Friday morning, the agricultural towns along the Tigris road quiet and domestic. The whole trip took ninety minutes each way. I ate lunch in a small place in Salman Pak — the modern town adjacent to the site — where the owner grilled lamb chops over wood and served them with rice and a bowl of fresh chopped onion and tomato. We communicated entirely through pointing and enthusiasm. On the drive back, the arch was visible in my rear mirror for a long time, still enormous, still there.

When to go: November through March. The site is entirely exposed with no shade except under the arch itself, and the Iraqi plain heat makes summer visiting genuinely dangerous. The drive from Baghdad is best done as a morning excursion, returning before the afternoon sun peaks.