The collapse was a stark reminder of the threat posed by extreme weather events, which are expected to multiply in the coming years under human-induced global warming.

August brought torrential rains to the Sahel, and to Mali's Tomb of Askia -- a 17-metre (55 feet) pyramid in the city of Gao, inscribed on UNESCO's list of World Heritage in Danger.

A section of the ceiling in the men's mosque -- which forms part of the fragile tomb complex -- gave way under the weight of the waterlogged earth.

"We hadn't had that amount of rain for 10 years", said Abdoulaye Cisse, an architect specialised in unbaked earth construction who is helping with the renovation project.

The work, which is being carried out by a few dozen local craftsmen, began in March four years after being announced.

Most has now been put on pause until the end of the rainy season in late September.

Cisse said the latest setback was to be expected, after many others before it.

"The structure is weakened, the wood is very, very old," he explained.

The tomb was built in 1495 by Askia Mohamed, the Songhai emperor who made Gao the capital of his empire with Islam as its official religion.

The four-hectare tomb complex, which includes mosques, a cemetery and an open-air assembly area, is the best conserved remnant of the empire's former power across West Africa, according to UNESCO.

- Unprecedented intervention -

The UN's cultural body listed the Tomb of Askia as World Heritage in 2004.

But eight years later, it was added to the in-danger list after Gao, like UNESCO-recognised Timbuktu, fell under the control of jihadists and separatist fighters.

UNESCO concluded that protection of the site could no longer be guaranteed.

The rebels have since been driven out of the northern towns, but the region is still plagued by violent unrest and foreign experts can no longer visit.

The new restoration project therefore came as welcome news.

Ali Daou, a programme manager at UNESCO, said he was "unaware of any intervention of this scale" since the tomb was built more than 500 years ago.

Traditional restoration happens roughly every two years, when locals gather to repair the banco render -- a mixture of clay and chopped straw.

But such constant alteration can be problematic.

The overloading of earth creates fragile sections, and the use of non-native wood to replace the local hasu variety compromises the building's authenticity.

Cisse said the new renovation project involved re-installing traditional wood back into the structure.

The team is also restoring the tomb's original shape, which had been "badly eroded by rain and wind", he added.

- 'Torrential rains, violent winds' -

The Sahel is one of the world's regions most impacted by global warming.

Along with the northern Gulf of Guinea, it has been hit in recent weeks by rainfall "120 to 600 percent above the average for the period 1991-2020", according to the Agrhymet food security organisation.

Hundreds in the region have perished in the flooding.

"It has been clearly established that climate change poses a direct and indirect threat to the tomb", said Daou from UNESCO.

"Directly because of torrential rains accompanied by violent winds and indirectly because of the increasing scarcity of husa wood".

The project is managed by the Malian government and funded to the tune of $500,000 by the Aliph Foundation, an international NGO for the protection of heritage in conflict zones.

The French association CRAterre is also providing expertise in raw earth construction.

The restoration champions sustainable practices, with a nearby nursery planted to supply the husa wood and the labour providing work to locals.

"One of the real issues is how traditional earthen architecture can cope with climatic phenomena that are likely to be more severe than in the past", said Valery Freland, the executive director of Aliph.

Both Cisse and Daou are confident that Gao's population will stay and protect the historic site despite the threat of desertification and conflict.

The masons know the tomb complex by heart since "they pray there five times a day", Cisse said, adding that "their vigilance can strengthen its resilience".