Here's why:
* Proper noun: When referring to a specific, named region, it's treated as a proper noun. For example:
* "We took a trip to the Amazon Rainforest, a famous South American jungle."
* "The Amazon Basin is a vast South American jungle."
* Common noun: If used as a general term, it's not capitalized:
* "They studied the plants and animals found in the south American jungle. "
In short, capitalize it when you're referring to a specific named area, but not when you're using it as a general term.