A cave system is actually not one single cave but a series of connected caves. The Mammoth Cave in Kentucky is the longest cave system in the world. The system is remarkably stable because it is made of limestone that is covered by a layer of sandstone. There are more than 405 miles of passageways that weave through the massive cave system. Every year, new caves and connections are discovered. Several North American tribes have been known to settle in the cave system throughout history. Scientists have been researching the people that had once called the cave system home. The remains found in the cave had been protected from the elements, which makes them great specimens for studying. They are so well preserved that scientists can easily study the people's dietary habits. This research has found that the people living in the cave were once hunter-gatherers, and then they transitioned to a community with domestic plants and agriculture. That means they learned how to grow their own food instead of having to hunt for it.

Researching in this type of environment is tricky business. Moving an artifact even a little can diminish its research value. That is why explorers are trained to not disturb artifacts they find. They learn how to collect and study the items they find without spoiling them. It is because of this sensitivity that the cave system is protected and is now a national park.

What evidence does the author use to support the idea that the cave system is stable?

a
A cave system is not one single cave but a series of connected caves.

b
Every year, new caves and additional connections are discovered.

c
It is made of limestone that is covered by a layer of sandstone.

d
There are more than 405 miles of passageways within the cave.