DAKAR (Reuters) – Natural and human factors made Sierra Leone’s capital vulnerable to a landslide that killed more than 400 people this week: heavy rain, deforested land and communities forced by overcrowding to live on steep hillsides.

Those vulnerabilities are mirrored in villages and cities across West and Central Africa – among the world’s poorest and wettest regions – that face a worsening threat from landslides, researchers say.

Hundreds are also missing after the side of Mount Sugar Loaf collapsed near Freetown on Monday in one of the worst flooding-related disasters in Africa in years. On Thursday, another landslide in a fishing village in eastern Democratic Republic of Congo killed at least 40.

Over 4,000 people have died in similar incidents over the past 10 years and millions of dollars of resources are lost because of them each year in Africa, according to Ogbonnaya Igwe, a senior lecturer at the University of Nigeria, who studies landslides.

“Triggered by intense rainfall, changes in water level and anthropogenic activities, it is very difficult … to overestimate their threat to public safety and the environment,” Igwe said in a report.

Landslides generally happen after periods of heavy rain, saturating or liquefying soil and causing slippage. They are more likely to occur if soil is laid bare by deforestation or urban planning.

That was the case in Sierra Leone and in other mudslides in recent years in Nigeria, Cameroon, Congo and Uganda.

An additional factor in Freetown was poor urban planning that put people unnecessarily at risk, according to aid groups and flood experts. Freetown’s population has also grown sharply in recent decades, raising pressure on living space.

Experts say the regional problems could be getting worse. The volume and intensity of rainfall in West Africa is increasing, Igwe said. Urban populations are on the rise and so is deforestation.

“Once you turn a forest into something else and the land is on a hillside, you increase the likelihood of landslide,” said Nelson Odume, a senior researcher at the Institute for Water Research at Rhodes University, South Africa.

Based on its geographical similarity to other areas that have suffered landslides, Igwe said that Conakry, the hilly, wet capital of Sierra Leone’s neighbor, Guinea, faces similar dangers.

“We have studied areas that have similar features as Conakry and we are able to say with some degree of confidence that it is at risk,” said Igwe.

Predicting a landslide is difficult. Sometimes a slow slippage or fissures in the ground offer a sign of what is to come. Mostly though they happen without warning.

Igwe uses satellite and GPS data to measure small ground movements to detect the possibility of landslides in vulnerable areas like the Adamawa highlands along the Nigeria-Cameroon border.

But, ultimately, the best way to lessen the danger is to stop the root causes, experts told Reuters, including by developing local economies and improving urban planning with the safety of residents in mind.

“They need to diversify people’s livelihoods,” Odume said. “No one will stop cutting trees down if that is all they have to do.”