But the latest renovation aims not only to restore the wonders of the 1,488-year gem, but to ensure it survives the next earthquake to hit the ancient city.

From afar, its dome, shimmering rock and delicate minarets appear to watch over Istanbul, as they have for centuries.

As visitors get closer however, they see scaffolding covering its eastern facade and one of the minarets.

While "the renovation of course breaks a little bit the atmosphere of the appearance from the outside" and the "scaffolding takes away the aesthetic of the monument... renovation is a must," said Abdullah Yilmaz, a guide.

Hagia Sophia, a World Heritage Site and Turkey's most visited landmark, "constantly has problems", Hasan Firat Diker, an architecture professor working on the restoration, told AFP.

That is why it has undergone numerous piecemeal reconstructions over the centuries, he added.

- 'Global' makeover -

The current makeover is the first time the site will undergo a "global restoration", including the dome, walls and minarets, he said.

When it was first completed in AD 537, on the same spot where previous churches had stood, the Hagia Sophia became known as a shining example of the architecture of the Byzantine Empire, which ruled the city known as Constantinople at the time.

It served as a church until the fall of the city to the Ottomans in 1453, when it became a mosque.

In 1935, Mustafa Ataturk, the founder of modern Turkey who forcibly remade the country into a secular one, turned the building into a museum.

It remained as such until 2020, when President Recep Tayyip Erdogan, a practising Muslim who came to power at the head of an Islamist-rooted party, turned it back into a mosque.

- Next big quake -

Like the residents of this historic city, the Hagia Sophia has not only had to contend with the whims of its rulers -- it faces the constant danger from earthquakes that have regularly struck the metropolis, the last major one in 1999.

Like many buildings in the city of 16 million, which lies just kilometres from an active seismic fault line, Hagia Sophia does not meet building earthquake standards.

Its dome collapsed in an earthquake in 558 and the building has been damaged in other quakes that have hit the city since.

So the main goal of the restoration under way is to "reinforce the building against the next big earthquake" so that the ancient structure "survives the event with the least damage possible," said Ahmet Gulec, a member of the scientific committee supervising the works.

For the moment specialists are studying the dome to determine how best to both reinforce and restore it, Diker said.

The interior is for now free of any scaffolding. But eventually four huge pillars will be erected inside to support a platform from where specialists will restore the dome's paintings and mosaics.

"Once you're inside... it's perfect," marvelled Ana Delgado, a 49-year-old tourist from Mexico as the hum of laughter, conversation and movement filled the building following afternoon prayers.

"It's magic," chimed in her friend, Elias Erduran, from the Dominican Republic.

- Millions of visitors -

Hagia Sophia saw 7.7 million visitors stream through its spacious interior last year.

Around 2.1 million of them are foreign tourists, many of whom pay 25 euros for an entry ticket, generating millions of euros annually.

Officials hope the inside pillars will not deter visitors from coming during the works, which are expected to last for several years. Officials have not said how much the renovation is expected to cost.

"The objective is that the visits and prayers continue" during the works, Gulec said.

And even if some visitors are disappointed not to have witnessed the building in all its glory, the important thing "is that one day my children will also be able to admire Saint Sophia," said Yana Galitskaya, a 35-year-old visitor from Russia.