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It takes a full day to climb up the side of the volcano. From the edge, two of the world's leading volcano scientists look down into the crater. It is black with hardened lava.

Dario Tedesco and Ken Sims know that the lava is melted rock that bubbles up from inside the volcano. After it cools, it becomes solid.

In the center of this blackened rock they see a stunning sight - a lake of boiling lava. The lake is 213 meters (700 feet) across. It is one of the largest lava lakes in the world. A splatter cone shaped like a giant soup bowl forms the sides of the lake.

Even from the top of the volcano, the scientists can feel the heat. The lava tops 9820 Celsius (1,8000 Fahrenheit). The liquid rock spews upward. It changes from orange to black in midair as it cools.

Seeing the lake is one thing. Hearing it is another. It roars like a jet plane taking off. The lava crackles and pops. A thick white plume of deadly gases shoots skyward.

Sims is awestruck. After a long silence, he points to the lake. "There," he says, "is where I'd really love to get a sample."

Sims, Tedesco, and their team have come to Africa on a three-week mission. They want to study this volcano. It is called Nyiragongo.

It is one of the most active volcanoes on Earth. It is also one of the least studied. A war that has lasted for nearly 20 years has kept scientists away.

At the base of the volcano lies the city of Goma. It is home to as many as one million people. The population there grows by the day. If the volcano erupts, Goma is in trouble. "Goma," says Tedesco, "is the most dangerous city in the world."

Goma sits on top of a 6,000-kilometer (3,700-mile) long crack in Earth's surface. Think of Earth's outer layer, the crust, as a broken eggshell. It is made up of more than a dozen large pieces called plates.

Beneath Earth's plates lies the mantle. This layer is hot enough to melt some rock and make it flow like molasses. Below the mantle is Earth's outer and inner core. The outer core is always liquid. The inner core is solid. The pressure at the inner core is so great, it cannot melt.

Earth's plates float on the liquid part of the mantle. They are constantly shifting and moving. Some bump into each other. Some slide past each other. Others pull apart.

The city of Goma is located where two of these plates move away from one another. When these plates shift, the volcano becomes more active. Some 13 kilometers (8 miles) below the city, magma boils in a large magma chamber. When the magma is forced to the surface, it shoots out of the volcano as lava.

The scientists worry about Goma. The question is not if the nearby volcano will erupt, but when. The volcano has erupted twice in the recent past.

In 1977, an eruption broke the walls of the lava lake. It took less than an hour for the lake to drain. Lava raced down the mountain at more than 95 kilometers (60 miles) an hour. This was the fastest moving lava ever recorded. The river of lava rolled through neighborhoods. It destroyed everything in its path.

In 2002, the volcano erupted again. This time, it spewed fountains of melted rock into downtown Goma. Lava destroyed thousands of homes. It buried buildings to the top of the first floor. People fled. Both eruptions were small, though, compared with what the volcano might do.

Protecting Goma is why the scientists climbed the volcano. They want to save lives. To do that, they need to learn as much as possible about the volcano. They need to take measurements. They need to collect rock and gas samples. Sims has his eye on something else, too. What he wants most is a "zero-age sample."

A zero-age sample is a chunk of fresh, hardened lava from the lake. A sample like this can help scientists tell the age of all the rocks they collect from the same location. "Aging" the rocks can help them make better predictions about when the volcano might erupt.

Getting a sample will not be easy. They must get it from the lava lake itself. Sims knows the lake is wildly unpredictable. Gurgling lava can shoot 18 meters (60 feet) into the air. These explosions can send waves of lava over the edge of the splatter cone. When the lava spills, it melts everything in its path. The closer Sims gets to that lake, the more he is at risk.

Most of the group will stay on top of the volcano where they have set up a small camp. Only a few scientists will climb down into the crater. They will have their work cut out for them. The hiking will be steep and the footing unsure. The team plans to go slowly and take breaks on a series of ledges inside the crater.

To climb down safely, each climber wears a harness around his or her legs and waist. Sims goes first. He anchors a rope into the rock. He clips his harness onto the rope and slowly steps down the crater's walls.

Sims kicks any loose rocks he sees and watches them tumble downward. Many crumble loose when he puts weight on them. Team members follow in a single file, keeping a safe distance between each other.

After hours of climbing down into the crater, the sun begins to set. The team makes camp on one of the wide ledges. From below, the lava lake glows an eerie orange in the night.

In the morning, the team sets up a camp laboratory. Before getting to work, each team member puts on a gas mask to protect his or her lungs.

The lava lake gives off tons of poisonous gases. In very little time, the team sees how dangerous the gases can be. Zippers rust. Camera lenses break down.

As the team begins to collect samples and data, Sims worries about the zero-age sample. He knows that getting it will be dangerous. He knows he would never allow another member of his team to risk his or her life for this sample.

He also knows that he has the scientific knowledge to get the right sample. He has the right climbing skills, too. He thinks of his family, including his two young children. He struggles to make a decision. Finally, he decides to take the risk.

Throwing his rope over the side of the ledge, Sims climbs down to the crater floor. From here, he can no longer see into the lava lake. The steep sides of the splatter cone rise up, blocking his view. They also shield him from the bubbling lava.

He hears the lake's hissing gases and smells its bitter fumes. He puts on a special suit. It protects him from the heat, but it's stiff and hard to move in.

Once dressed, he shuffles to the splatter cone wall. The hardened lava under his shoes crunches like eggshells.

He looks for places to hold on to as he climbs up. He stretches his arms and legs across the rock to reach them. He is drenched in sweat. He is nearly at the top when his foot slips. He smells burning rubber. Looking down, he sees that one of his shoes is melting.

Sims keeps climbing. He peeks over the top. He sees the boiling lava below. This is his chance. He needs to get a sample.

He reaches forward and slams his fist against the top of the splatter cone. A small piece of freshly hardened lava breaks off. It is shiny and black and hot. Even with his gloves on, he juggles the hot rock from hand to hand.

Sims scrambles back down the face of the splatter cone. He has the zero-age sample. To get it, he traveled up a mountain, down a crater, up a splatter cone, and to the edge of a lava lake.

He risked his life. Yet what he holds in his hands could be the key to saving Goma. It could also help him understand how other volcanoes work. To Sims, it was worth the risk. Having the sample isn't enough, though. He looks at the smoldering rock in his hands and thinks: Now it's time for us to get to work.

Article by Michael Finkel. Photographs by Carsten Peter