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It takes a full day to climb up the side of the volcano. Dario Tedesco and Ken Sims stand on the edge of the crater. They are volcano scientists. They look down and see an amazing sight. Inside, a lake of lava crackles and pops. It is one of the largest lava lakes in the world.

The lava is melted rock. It comes from deep inside the volcano. It bubbles and boils. It roars like a jet plane taking off. The lava shoots upward. It changes from orange to black as it cools.

Sims points down at the lake. "There," he says, "is where I'd really love to get a sample."

They call this volcano Nyiragongo. It's one of the most active volcanoes on Earth. The city of Goma sits at the base of the volcano. When the volcano erupts, the city is in great danger. That's why these scientists have come here. They want to protect the city.

Beneath Goma is a crack in Earth's crust. This outer layer of Earth is broken into large pieces. The pieces are called plates.

Beneath Earth's plates lies the mantle. It is made up of mostly magma, or liquid rock. Under the mantle is Earth's inner and outer core.

Earth's plates float on the liquid part of the mantle. They constantly shift and move. Some bump into each other. Some slide past each other. Other plates pull apart. Goma sits on plates that are pulling apart. When these plates shift, magma can shoot out as lava.

When the volcano erupted in 1977, the walls holding the lava lake broke. A river of lava raced down the volcano. It flowed through the city. It destroyed everything in its path.

In 2002, the volcano erupted again. Lava set fire to thousands of homes. It buried buildings. Yet these were only small eruptions. Future eruptions could be much bigger. They could do much more damage. That's why the scientists worry about Goma.

When will the volcano erupt again? Sims and Tedesco want to know. To find out, they need rock and gas samples from the volcano.

What Sims wants most of all is a "zero-age sample." A zero-age sample is a chunk of freshly hardened lava. The sample will help Sims predict the next eruption. There's only one place to get it. It must come from near the lava lake. Getting it will be hard. Gurgling lava can shoot into the air. Sims will be in great danger.

Sims' small team starts to climb into the crater. They each wear a safety harness. They move very slowly.

Sims goes first. He attaches a rope to the rock. Then he clips his harness onto the rope. Slowly he steps down the volcano's walls. His team follows in a single file. Loose rocks fall away beneath their feet.

They climb down until the sun begins to set. Then the team stops. They set up camp on a ledge. The lava lake glows fiery hot below them.

In the morning, Sims and the team get to work. They put on gas masks. The masks will protect them from toxic gases that come from the volcano.

Then they begin to collect samples. Sims thinks he is the best one to get the zero-age sample. He is a good climber. He can find the right sample, but it will be dangerous. Sims is worried. He thinks about his family. His children need him. Should he do this? In the end, he decides he must.

Sims throws his rope over the ledge. He climbs down to the crater floor. Here, he can't see the lake. The steep sides of the lake's walls block his view. He can hear the hissing gases. He can smell the bitter fumes. He can feel the terrible heat.

Sims knows he needs protection. He puts on a special suit. It will protect him from the heat. Now he begins to climb up the lake's wall. His foot slips. He smells burning rubber. One of his shoes is melting.

Sims keeps going. He peeks over a ridge of freshly hardened lava. The boiling lava lake bubbles below. Steady now. Steady. He slams his fist into the hardened lava. A piece breaks off. He grabs his sample. Success!

Sims risked his life, but it was worth it. What he holds could save many lives. He looks at the hot rock in his hands. It isn't enough. Days of hard work still lie ahead.

He starts to climb back up. He thinks, "Now it's time for us to get to work."

Article by Michael Finkel. Photographs by Carsten Peter