what was the Aztec capital that had been build upon an island in the middle of Lake Texco in central Mexico? It was connected to the mainland by broad causeways.

Tenochtitlán, which became the center of modern Mexico City.

The Aztec capital that had been built upon an island in the middle of Lake Texcoco in central Mexico was called Tenochtitlan. It was connected to the mainland by broad causeways.

The Aztec capital that was built upon an island in the middle of Lake Texcoco in central Mexico was called Tenochtitlan. To find the answer to this question, you could have searched for information about the Aztec capital or the geography of ancient Mexican civilizations.

Here are the steps to search for this information on the internet:

1. Open an internet browser of your choice.
2. Go to a search engine such as Google, Bing, or Yahoo.
3. Type in the query "Aztec capital built on an island in Lake Texcoco in central Mexico" and hit the Enter key or click on the Search button.
4. Look through the search results for relevant information.

In this case, you would find that the answer is Tenochtitlan. Additionally, you would also discover that Tenochtitlan was connected to the mainland by a system of broad causeways.