90 people were hospitalized, and a 12-year-old boy died.

Soon, Russian officials identified what had killed him.

The deadly infectious disease, anthrax.

The outbreak had started among reindeer.

To contain the spread, they burned over 2,000 reindeer carcasses.

The strange thing was, there hadn't been an anthrax outbreak in the area for more than 70 years.

So, to figure out where it came from, scientists started looking underground.

In the coldest parts of the world, there's a layer of the Earth that stays frozen all year.

Every summer, the soil above it thaws, but this deeper layer stays hard as rock.

This is permafrost.

Most permafrost is here, in the Northern Hemisphere around the Arctic.

And because it never thaws, permafrost acts kind of like the freezer in your kitchen.

When plants and animals here die, they don't actually decompose.

Instead, they become preserved in the frozen earth, like a time capsule.

And it's been that way for thousands of years.

But that's changing.

Today, humans are burning carbon and making the atmosphere warmer.

And that's causing the permafrost to thaw and shrink.

By 2100, only these areas will still have any permafrost.

And that's causing some problems.

When permafrost melts, the land above it becomes unstable, which can lead to landslides.

Man-made structures start to fall apart, as the ground underneath them collapses.

And dead plants and animals that had been frozen for years are starting to thaw out.

As they're exposed to air and bacteria, this organic material starts to decompose.

That releases greenhouse gases like carbon dioxide and methane.

But that's not all it releases.

In Siberia, scientists think that the anthrax outbreak came from a long-dead reindeer carcass that thawed out along with the permafrost.

Anthrax spores from the carcass would have spread across the area and infected reindeer grazing nearby.

And it's not just anthrax.

Scientists worry that, as permafrost melts, it could unearth all sorts of diseases we thought we had under control.

35 million people live around permafrost.

But the carbon released, as permafrost melts, will accelerate the impacts of climate change everywhere.

Rising seas, heat waves, droughts in some places, and floods in others.

And now we can add one more thing to that list, diseases we thought we conquered.