Some dynamite and a plastic bottle. That’s all it takes for a fisherman to kill hundreds of fish and transform thriving coral reefs into rubble in a matter of seconds.

Around the world, fishermen are using explosives, often with dynamite, to maximize their catch. Called blast fishing or dynamite fishing, the practice goes on in nations from Lebanon and Malaysia to the Philippines, while some countries—Kenya and Mozambique, for instance—have managed to stamp it out.

In Africa, Tanzania is the only country where blast fishing still occurs on a large scale—and it’s happening at unprecedented rates. “I would say probably for the last five years it’s at least as bad or worse than it’s ever been,” said Jason Rubens, a marine conservationist with World Wildlife Fund’s Tanzania branch.

In December, Wildlife Watch wrote about blast fishing after researchers from the Wildlife Conservation Society released a report documenting the extent of the illegal practice in the Indian Ocean off Tanzania. The researchers counted more than 300 explosions in 30 days, from the Kenya-Tanzania border down to Mozambique. That’s at least 10 blasts a day.

Now new video footage for National Geographic by reporters Sophie Tremblay and Willy Lowry captures some of the blasts in real time. In the film, in broad daylight just off the coast of Dar es Salaam, Tanzania’s biggest city, fishermen toss overboard bombs that blast skyward huge plumes of water. “It was just so brazen,” Lowry says, noting that the video shows only a fraction of the blasts he and Tremblay witnessed.

Why do they do it? As one dynamite fisherman in the video puts it: “The easiest way for people to earn something is from fishing with dynamite.”

Making a bomb is cheaper and vastly more productive than sticking with traditional fishing methods such as basket traps and hook and line. Dynamite is also easier to find nowadays. A boom in mining and construction in Tanzania has made it less challenging for people to get their hands on explosives.

But the practice has disastrous consequences. Off the coast of Dar es Salaam, fishermen aim for lucrative tuna, Tremblay says. But the blasts destroy not only their targets but entire coral reefs, which support fish, crabs, and other species and play a crucial role in controlling carbon dioxide levels in the ocean.

“It’s the three-dimensional structure that really provides a lot of habitat and space for fish eggs and larvae and juveniles to hide from predators and as a feeding habitat,” Rubens explains. “Blasting literally physically destroys the three-dimensional structure of the reef.”

Aside from the environmental impact, dynamite fishing threatens the livelihood of legitimate fishermen, as well as the economy as a whole. If people are too afraid to swim in the sea because of incidents like this, that’s a problem for Tanzania, where tourism constitutes 17 percent of the gross domestic product.

Check out the film to learn more about the dangerous fishing tactic and see blast fishermen in action.