Image caption Kumar, one of the Sherpas who died at Base Camp

The BBC's Tom Martienssen was 20,000 feet up Mount Everest when an earthquake killed 18 climbers. He returned later to witness the devastation in Nepal and to meet the families of Sherpas who died on the mountain.

It's 11:50 on 25 April when the Earth starts to move beneath my feet.

There are 500m-deep crevasses immediately behind me, three of the world's tallest mountains in front - and a crackling rumble accelerating through the mist, getting rapidly closer.

I am at Camp One on Everest in the middle of an earthquake and avalanches are starting to crash down the mountain.

I cannot help thinking: "People care about me, and because I'm on this mountain I will die. I'll be ruining the lives of those I care about so much."

In the next 10 seconds I believe I shall be no more.

"Stay calm, stay calm. Prepare to run!" my guide Rob Casserley's words reach me through a curtain of fear. I am truly petrified.

I survive, but 18 climbers are killed on the mountain below me.

Media playback is unsupported on your device Media caption Joe French: ''I didn't realise it was an earthquake to begin with''

Later I discover that three of our group's Sherpas are among them - killed when an avalanche ripped through Base Camp.

The earthquake has killed thousands more below them. Whole villages have been wiped off the map in remote areas of Nepal - many covered by landslides - and some neighbourhoods of the capital Kathmandu have been reduced to rubble.

I am trapped on the mountain for 48 hours before a helicopter takes me back to Base Camp, where the bodies of the dead lie covered by orange canvas. I have to go back to the UK but there's one thought on my mind: I want to meet the families of the dead Sherpas left behind.

Image caption Base Camp after the avalanche

The relief operation begins almost immediately.

Find out more Watch Thomas Martienssen report for Panorama, Disaster on Everest, at 20:30 on Monday on BBC One, or afterwards on the BBC iPlayer (UK only)

A team of British Army Gurkhas, who had been climbing the mountain to mark their Brigade's 200th anniversary, organised the rescue of those at Camp One, including me.

The Gurkhas then started taking aid to some of the remotest villages in Nepal.

After three weeks in the UK, I return to meet up with the most experienced soldier in the Gurkha team, Warrant Officer Govinda Rana. He has served for 23 years and had been due to be commissioned as an officer on the summit of Mount Everest.

We drive for two days along potholed roads and tracks. There's scarcely a building along the way that hasn't been damaged. Our destination is a village called Priti - this is the first time that aid has reached it since the disaster struck.

A small group of Gurkha engineers are building temporary classrooms for a school that is no longer safe.

A recce team is sent further into the wilderness to check on the 1,400 houses in the surrounding area. The results are staggering - not a single one is safe to live in. Water supplies are failing and a bridge has collapsed.

The following day the team hand out CGI - a corrugated steel building material - to enable villagers to build shelters for themselves and protect their families from the coming monsoons.

There is not enough to go round, however. As the soldiers are working with supplies provided by the Gurkha Welfare Trust, they give the materials to the families of serving or former Gurkhas. Other families will have to wait longer for help.

The Gurkhas have also taken steps to improve the water supply, however, and this benefits everyone.

It's time for me to move on.

My next destination is the home of one of the dead Sherpas, in the village of Cheskum - a place so remote that helicopter is the only realistic way for me to get there.

They farm by hand, they mill their own flour and they speak a dialect only the local teacher can translate.

Kumar, the dead Sherpa, never stopped smiling. His widow, Pancha, never stops working. There are four children.

They should be all right financially, as climbers who knew and worked with Kumar are supporting Pancha, and will pay for the children's education.

But a deep, entrenched sorrow is written into the lines of Pancha's face and the eldest daughter breaks down in tears at regular intervals throughout my stay.

Media playback is unsupported on your device Media caption Tom Martienssen meets the family of Sherpa Tensing Bhota

Flying back to Kathmandu the evidence of shattered communities is clearly visible. Brown lines of landslides, kilometres-long, break up the deep-green terraced fields. Whole villages are buried.

In the city, I go in search of the family of another of the Sherpas who died on the mountain. Tensing had spent 10 years helping Westerners like me to climb Everest.

Rounding a corner and ducking through a tight alley I reach the family's tiny rented house.

Tensing's widow, Pema, lives with their two boisterous boys. For my visit she has bought a bottle of cola, which I share with the children. She tells me of her hardships. She relied on Tensing for so much more than money.

The boys will grow up without a father. She may have to sell the land in her home village to pay for their education, she says. When I ask if they may want to climb, like their father, it reduces her to tears: she is desperate for them to get an education and a chance to work abroad.

In the following days she plans to return to her village. This means a day's bus ride, then a five-day walk from the end of the road.

Pema is ethnically Sherpa, as was Tensing, and part of their belief is that the soul passes on 49 days after death. It's an occasion marked with a celebration, which can often be very costly.

Pema will receive $15,000 from an insurance company. But how much of it will be left on the 50th day is one of the things I worry about as I leave the country.

It's not the only thing on my mind. The monsoon is coming - there will be three months of rain and hundreds of thousands of people still have nowhere to live. I admire the resilience of the Nepalese people, but I can't help but think the toughest challenges may still be to come.

Image caption Thomas Martienssen photographs fellow climbers hours before the avalanche struck

Watch Thomas Martienssen's report for Panorama, Disaster on Everest, at 20:30 on Monday on BBC One, or afterwards on the BBC iPlayer (UK only)

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