What happens when a city sits below a glacier, between steep mountains or beside a river with nowhere to escape? In some places, the answer can be catastrophic. Geography, geology, rapid construction and extreme rainfall can combine to leave thousands—or even millions—of people exposed.
The recent disaster in the Himalayas has put that risk back in the spotlight. But Nepal is not an isolated case. Around the world, cities are growing in places where mountains, rivers and unstable slopes meet. Climate change is adding another layer of uncertainty by increasing the likelihood of extreme rainfall and accelerating changes in some high-mountain environments.
Here are 10 cities where those warning signs are particularly difficult to ignore.
1. Kathmandu, Nepal
Nepal’s capital sits inside the Kathmandu Valley, surrounded by hills and drained by the Bagmati River and its tributaries. More than three million people live in the wider urban agglomeration, concentrating a huge population in a relatively enclosed basin.
Then there is the earthquake risk.
The 2015 magnitude-7.8 Gorkha earthquake triggered more than 21,000 landslides across Nepal. Many slopes were left weakened and vulnerable to later rainfall.
That creates a dangerous sequence. An earthquake can fracture rock, monsoon rainfall can saturate it, and a subsequent landslide can push huge quantities of sediment into waterways.
Climate change could make the rainfall part of that equation more difficult, particularly as extreme precipitation becomes more intense.
2. Gangtok, India
Gangtok, the capital of Sikkim, is perched at around 1,650 metres above sea level on steep Himalayan slopes. Unlike cities built on broad plains, Gangtok has very little level ground.
The town lies within the wider Teesta river basin, where narrow valleys can channel water and debris rapidly downhill.
There is another concern: Sikkim is part of the seismically active Himalayan belt. An earthquake can weaken slopes, while heavy monsoon rain can later mobilize the fractured material.
And the high mountains provide another potential source of trouble. Sikkim contains glaciers and glacial lakes, meaning hazards can originate far above the city before travelling through steep valleys.
The combination makes Gangtok a classic example of why climate change, earthquakes and mountain hydrology cannot always be considered separately.
3. Dehradun, India
Dehradun is the capital of Uttarakhand and lies in the Doon Valley between the Himalayas and the Shivalik Hills. The district has around 1.7 million residents, according to government figures.
The surrounding mountains funnel rainfall toward the valley. During intense storms, streams can rise quickly, while saturated slopes become more susceptible to failure.
Urban growth adds another complication. Roads, buildings and altered drainage patterns can change where rainwater travels.
A landslide does not necessarily have to hit the city itself. If it deposits enough rock and soil into a stream, the channel can become partially blocked or its capacity reduced by sediment.
Climate change matters because a short burst of extreme rainfall can overwhelm systems designed around historical rainfall patterns.
4. Quito, Ecuador
Quito stretches through a narrow Andean valley beneath Pichincha volcano. Instead of having one large river channel, the city is intersected by hundreds of steep ravines known as quebradas.
These natural channels are effectively pathways for mountain runoff.
During intense rain, water can move rapidly through them. If a slope collapses into a quebrada, the flow can suddenly become loaded with mud, boulders and vegetation.
Quito also lies in the earthquake-prone Andes, providing another mechanism for destabilizing slopes.
The city therefore has three important ingredients: steep terrain, confined drainage channels and seismic activity. Add increasingly intense rainfall associated with climate change, and the potential for cascading hazards becomes more serious.
5. Medellín, Colombia
Medellín occupies the narrow Aburrá Valley, with steep hillsides rising immediately around the metropolitan area.
The city’s rapid expansion has pushed development onto slopes where landslide exposure is greater. That vulnerability became painfully clear in June 2025, when a rainfall-triggered landslide in Granizal killed 27 people, destroyed dozens of homes and displaced more than 1,000 residents.
The geology matters too. Parts of the eastern valley contain weathered rock formations that are susceptible to mass movement.
Once a slope fails, the material can enter streams and drainage channels. The mountain does not have to fall on the city to cause a citywide emergency.
Climate change becomes important when heavier rainfall pushes more water into already vulnerable slopes.
6. Rio de Janeiro, Brazil
Rio has more than six million residents within the municipality, many living remarkably close to steep mountains.
The city’s famous granite peaks and forested hills rise directly behind densely developed neighborhoods. That creates a simple but dangerous physical relationship: water falling high above the city can reach the streets very quickly.
Heavy rainfall has repeatedly caused landslides and flooding. Informal settlements on vulnerable hillsides can increase exposure, particularly where construction alters natural drainage.
When soil, rock and vegetation enter rapidly flowing runoff, the result can be a debris flow capable of damaging roads, buildings and drainage infrastructure.
Climate change adds pressure through the possibility of more intense rainfall episodes, even if the annual rainfall total does not change dramatically.
7. Taipei, Taiwan
Taipei lies in a low-lying basin surrounded by mountains and drained by major rivers including the Keelung, Xindian and Tamsui.
The city had approximately 2.4 million residents in 2026, packed into a relatively small urban area.
Its surrounding mountains receive intense rainfall from typhoons, sometimes in extraordinary volumes over only a few hours. Saturated slopes can fail, sending sediment into streams that ultimately drain toward the basin.
Taiwan is also highly seismic. An earthquake can crack and weaken slopes, leaving loose material that a later storm can mobilize.
Climate change makes extreme rainfall the variable to watch, particularly as warmer air and oceans can contribute to more intense precipitation events.
8. Huaraz, Peru
Huaraz may be the most striking example on this list.
The Peruvian city sits below the Cordillera Blanca, home to numerous glaciers and glacial lakes. Above Huaraz is Lake Palcacocha, which contains roughly 17 million cubic metres of water and is fed by the retreating Palcaraju glacier. (frontiersin.org)
The lake has a terrifying history.
In 1941, a failure of its natural moraine dam released more than 10 million cubic metres of water, devastating Huaraz and killing nearly 2,000 people.
Today, the lake is substantially larger.
Scientists have linked its expansion to glacier retreat influenced by human-caused warming, increasing the potential consequences of an outburst.
A major rock or ice avalanche entering the lake could displace water and potentially trigger a catastrophic flood.
This is climate change creating a bigger hazard above a growing city.
9. Freetown, Sierra Leone
Freetown sits on the mountainous Freetown Peninsula, where steep slopes rise directly behind densely populated areas.
The city has already experienced the kind of cascading disaster this article examines.
In 2017, days of heavy rainfall preceded a massive landslide near Regent. Around 300,000 cubic metres of debris entered a flooded valley before moving roughly six kilometres through a drainage channel toward the coast. About 1,100 people died.
The disaster demonstrated how quickly sediment can transform flowing water into a destructive debris flow.
Deforestation and development on vulnerable slopes have added to the city’s exposure.
Climate change raises concern because extreme rainfall is one of the key ingredients in this chain.
10. Port-au-Prince, Haiti
Port-au-Prince faces an unusually complicated mix of earthquakes, steep hills, deforestation, flooding and rapid urban growth.
Haiti’s 2010 earthquake destabilized enormous areas of land, while vegetation loss has reduced natural protection against erosion in many locations.
Heavy rainfall can then mobilize loose soil and sediment, particularly on steep terrain.
The country’s exposure is enormous: the World Bank estimates that more than 96% of Haiti’s population is exposed to hurricanes, floods or earthquakes.
For Port-au-Prince, the risk is therefore cumulative. A disaster does not need to arrive in isolation. A weakened slope, followed by extreme rainfall, can create a completely different emergency.
The Bigger Picture
These are not predictions of the next disaster. They are cities where geography has already created a difficult starting point.
Climate change can add another variable by influencing extreme rainfall, glaciers and other natural systems. In places where mountains tower above dense populations, that extra pressure deserves attention.
Sometimes the biggest warning sign is not the rain.
It is what is waiting above the city.