Who are Los Topos, Mexico's volunteer rescuers?
They were formed spontaneously in response to a huge earthquake in 1985. Since then, Mexican volunteer rescuers known as the Topos - Moles - have helped in earthquakes from Iran to Nepal. Now, they are needed back at home.
The devastating 1985 earthquake struck on 19 September, the very same day as the one this year. It flattened 30,000 buildings in Mexico City and killed an estimated 10,000 people.
The Topos volunteers began to lift rubble, look for signs of life and pull people out, assisting the stretched and overwhelmed emergency services.
The next year, there was an earthquake in El Salvador and some of Los Topos headed south to help the neighbouring country in need.
They have since earned a reputation worldwide, and have assisted after earthquakes in Japan (1995), Colombia (1999), Haiti (2010) and Nepal (2015), among others, and in Indonesia after the 2004 tsunami.
They travel with small tools - pickaxes and saws, for instance.
Now their skills are in need at home again. Although many people have come together to assist with the earthquake rescue, these groups are different; they have specialised knowledge, as they keep up training throughout the year in the earthquake-prone country.
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They come from many professions but are still an unpaid force, and they work closely with firefighters and paramedics. They receive regular training in first aid, moving through and around collapsed buildings, climbing, managing search and rescue dogs, and fighting fires.
Their signal is well known in Mexico: Arms in the air, balled fists.
It means: Be quiet. We think we hear someone.
Asking for silence like that dates back to the earthquake in 1985. One of the rescuers later told the BBC (in Spanish): "I remember that the volunteers formed human chains and I watched with curiosity how they asked for silence to try to listen to the people who had been trapped.
"The government was overwhelmed," the rescuer, Eduardo Acevedo, continued.
"We had to organise ourselves.
"When people saw that this small group was taking people out of the rubble, they started calling us moles.
"We were the only ones who were ready to act in situations of risk."
On the website of one of the two groups that now carry the Los Topos name, they say: "To be a Topo, all you need is to want to help people, be tenacious and brave, and have the disposition to want to protect and help."