Sea cucumbers might not look it, but they are very valuable creatures — a kilo can cost over $3,000.

They are so prized that people will risk their lives to get ahold of one. The rarer these animals get, the deeper divers have to swim to find them.

There are 1,250 different species of sea cucumber in the world. Watch the video above to learn what makes them so expensive.

Narrator: Cucumbers usually cost under $3 a kilo. But sea cucumbers can set you back over $3,000 a kilo. In fact, they're so valuable people will risk their lives to get ahold of one. They might not look it, but sea cucumbers are pretty special creatures. Just ask this guy, Steven Purcell, one of the world's foremost experts on sea cucumbers.

Purcell: They're quite strange animals. They don't have any limbs, they don't have any eyes. They have a mouth and they have an anus and a whole bunch of organs in between.

Narrator: These otherworldly animals have been prized as a delicacy in Asia for centuries, where the wealthiest class would eat the animals as a nutritious high-protein treat. But it wasn't until the 1980s that demand exploded. A growing middle class in China meant more people could afford the luxury. Today, they're typically dried and packaged in ornate boxes, then given as gifts and served on special occasions. So, the fancier and more unusual-looking, the better. And more expensive. It turns out...

Purcell: The spikier the animals, the higher the price.

Narrator: And of the 1,250 different species of sea cucumber in the world, the Japanese sea cucumber takes the cake.

Purcell: Imagine some sort of mystical dragon slug with all these sort of spikes coming out of it.

Narrator: At up to $3,500 a kilo, it's the most expensive sea cucumber on the market. Compared to other varieties, like the Golden Sandfish, Dragonfish, and Curry Fish. And even if you order a common species on Amazon, you could still pay over $170 for a plate. Besides presentation, cucumber connoisseurs also value thick, chewy bodies, and to a lesser extent, taste. But the experience of eating them is only part of their appeal. Turns out sea cucumbers contain high levels of a chemical called fucosylated glycosaminoglycan in their skin, which people across Asia have been using to treat joint problems like arthritis for centuries, and more recently in Europe, where people are using it to treat certain cancers and to reduce blood clots. The sea cucumber craze now comes from all sides. You have the original Asian delicacy demand that started in the 1980s, and the new interest from Western pharmaceutical companies. In response, nations have clamored to harvest their local species. From Morocco to the United States to Papua New Guinea, everyone wants in on the sea cucumber trade.

Purcell: It's just spread like a contagion from one country to another.

Narrator: For example, from 1996 to 2011, the number of countries exporting sea cucumbers exploded from 35 to 83. But unfortunately, sea cucumbers couldn't handle the strain. In Yucatan, Mexico, for example, divers saw a 95% drop in their harvest just between 2012 and 2014, and that's a problem for everyone. For one, because the more sea cucumbers are harvested, the rarer and more expensive they become. Average prices rose almost 17% worldwide between 2011 and 2016. And the rarer these animals get, the deeper divers are swimming to find them. That's when fishing gets dangerous.

Purcell: Some countries, they're doing that without either a lot of training. In some of the tropical countries, you're getting a lot of people either becoming paralyzed through decompression sickness.

Narrator: So far, at least 40 Yucatan divers have died trying to harvest sea cucumbers. And as demand continues to increase, the problem is only getting worse. Of the 70 or more species of exploited sea cucumbers, 7 are now classified as endangered, all through exploitation, forcing numerous fisheries worldwide to shut down and damaging local economies in the process. So, why not farm sea cucumbers and leave the wild ones alone? Well, it's easier said than done, since many larvae die before reaching maturity, and those that do survive take two to six years to grow to a marketable size. That said, aquaculture for a few varieties has started to take off. Like with that fancy Japanese sea cucumber.

Purcell: There's now aquaculture in China in the billions.

Narrator: Hopefully more species will be farmed instead of fished in the future, if not to protect local economies and help develop potentially life-saving drugs, then at least to preserve a fascinatingly bizarre animal.