By Duncan Kennedy
BBC News, Mexico City
The world of smuggling covers a wide range of goods.
Botanists warn poaching is threatening Mexico's floral heritage
Drugs, guns, alcohol, tobacco. You name it.
But in Mexico, organised crime has moved into a different market: cactus plants.
It has become a multi-million dollar business, so sought after are some of the species.
To understand why, it helps to meet Doctor Leia Sheinvar.
An expert in cacti for 40 years, she leads invited guests into her special, locked room in Mexico City.
Behind the padlocks and keys is a unique world of cacti, protected because they are so rare and so valuable.
The colours range from green to pink to blue; some plants are as tall as a tree while others, like one Dr Sheinvar picks up, are the size of a baby's finger - Ariocarpus kotschoubeyanus.
"Some time ago," she says, looking at the plant, "this cactus sold in Europe. Weight for weight, it was more valuable than gold."
There are about 2,000 species of cacti, a plant native to the Americas.
Mexico is home to just under half (46%) of the species - plants like Aztekium hintonii, a small globular cactus and Astrophytum asterias, which rejoices in common names such as sea urchin cactus, sand dollar cactus and star peyote and which has long been on the endangered list.
Mexico is home to hundreds of species of cacti
The prickly, but delicate, resilience of cacti has moved songwriters and artists for years, but now they are motivating a less welcome presence - organised crime.
Adrian Reuter, of the WWF, spends his life tracking cactus smugglers. He has heard of some rare species being trafficked out of Mexico to buyers willing to pay $10,000 (£5,000) for a single plant.
"They are so profitable for such little work," says Mr Reuter. "The risks for organised crime are less and the profits are great."
The rise in cacti poaching has been fuelled by private collectors and the growth in the US of the practice of adapting gardens to dry landscapes to lessen the need for irrigation.
Researchers say the problem is now so serious that they no longer publish the locations of newly-discovered species in order to keep the smugglers at bay.
Mr Reuter showed us some photos of cactus smugglers at work. One image is of three men with shovels prizing a giant round cactus from its desert floor.
Another is of a suitcase full of smaller species wrapped in paper. The case was impounded by customs officials at Mexico City's international airport.
Mr Reuter told us about a street market where protected species of cactus are sold illegally.
We went to look for ourselves and secretly recorded.
After visiting a few stalls, we reached one where a woman showed us her plants.
We had been told what species to look out for and one appeared to be suspect.
Prosecutions for plant poaching are rare
If found out, this woman could face a fine of $25,000 or two years in jail.
But here in Mexico, the law is often not backed up by law enforcement.
We took our findings to Salvador Arias, a cacti expert at Mexico's National Autonomous University, Unam.
He soon confirmed that the species we had seen were taken from the desert and had not been cultivated under licence.
"It is clear to me they are illegal," he said. "You can tell the smuggled plants. They tend to be more damaged and come without that perfect look you find in garden centres."
The authorities now believe smuggling of cactus plants is the third biggest racket in Mexico, behind drugs and guns.
But prosecutions are rare. It is believed the last one was two years ago. The police are, instead, concentrating their resources on the country's drug cartels.
Vast, vacant, desert landscapes in Mexico are now becoming very lucrative.
"Does it really matter if a few plants are taken?" I asked Mr Arias.
"Yes," he replies. "It is not a few plants and they are often on the point of extinction. It is sad that Mexico's floral heritage is being squandered for money. Once these plants are gone, they're gone."