Where did this well known symbol come from?

In order to further explore a cave in San Antonio, Rob Nelson crawls through incredible tight caverns. Don’t worry though, he’s only a little claustrophobic.

There’s a mysterious underwater rock circle that supposedly has carvings in it.

Rob's gear malfunctioned underwater.

Go with Rob behind the scenes to see where parts of the show are shot!

Gungywamp was one of Rob’s favorite spots during this season of Secrets of the Underground!

The Great Serpent Mound is a 1,348-foot-long prehistoric effigy mound in Ohio. Scientists are sure what it’s purpose was.

A collection of dinosaur marks can be found near Denver. Some of these markings are believed to be part of a mating ritual that can still be seen in modern bird species.

Enormous dinosaurs and teeny birds have a lot more in common than you might think!

First, scientists like to date the rock in which the fossil is on. Radiometric dating is a technique used to date materials such as rocks or carbon.

What is Pi and where did its symbol come from?

The cave in central Texas is home to more than 10,000 prehistoric fossils.

A single bill contains dozes of symbols.

The official reports claim that all 57 workers died from Cholera, but something more sinister may have occurred.

Funny enough, these symbols were used to classify plants before classifying humans and animals.

In this digital exclusive, Rob Nelson shares extra details about his time spent in this cave full of prehistoric fossils.