EL CALLAO, Venezuela (AP) — Venezuela is known to have the world’s largest oil reserves, but its soil holds another valuable resource: gold.

The government in 2016 established a huge mining development zone stretching across the middle of Venezuela to diversify its revenue. Seven years later, there is a proliferation of mines burrowing for gold, diamonds, copper and other minerals.

The Mining Arc of the Orinoco is plagued with violence and shrouded in secrecy because many mines operate outside or on the margins of the law. They offer lucrative jobs for ordinary Venezuelans, but conditions are brutal.

At an underground mine in Bolivar state, operators use dynamite to loosen rocks some 260 feet ( 80 meters) below the surface, where workers descend daily to toil in oppressive heat with no safety gear.

The miners typically begin their day strapping themselves to a thick steel wire, holding on as best then can while dropping about 200 feet (60 meters) down a shaft, entering a world where headlamps provide the only light. They wear shorts and flip-flops or rubber boots and must bend at the waist to walk 60 feet (20 meters) down a quasi-ramp. There, they collect rocks and throw them into sacks to be carted via pulleys above ground to a grinding mill.

One of the miners, Alfredo Arriojas, says he doesn’t like mining, but has been doing the job for more than two years in hopes of owning a home, with money left to “invest it in something good that gives me income.”

By law, about half of the extracted gold must enter state coffers, but authorities as well as critics of the government report growing illegal mining. Rights advocates say that labor laws are flouted and that human rights violations abound. Violence between rival gangs prompts many miners to reconsider their trade.

Another nearby mine in Bolivar captures gold through open-pit operations on the surface, where workers spend hours near ponds that breed mosquitoes that transmit diseases such as malaria.

Open-pit miner José Rivas says he’s had enough: “I just want to buy my house and work on something else.”