The correct answer is "I am learning about Japan—believe it or not, it is a chain of around 4,000 islands"
Step-by-step explanation:
The dash or em dash (—) is a punctuation mark that can replace others including parentheses or commas as its main function is to set off information that is additional or that is used to emphasize previous ideas, because of this in the case the information that is additional or emphasize another idea is in the middle of the sentence an em dash is placed before and after it but in the case this information is at the end only one em dash is required. Considering this, in the case of the sentence "I am learning about Japan believe it or not, it is a chain of around 4,000 islands" the information that emphasizes the idea "I am learning about Japan" is "believe it or not, it is a chain of around 4,000 islands" and therefore an em dash needs to be placed before the word "believe" and thus, the correct sentence is "I am learning about Japan—believe it or not, it is a chain of around 4,000 islands".