Please answer a question about the following article about Houston:

Houston's economy has a broad industrial base in energy, manufacturing, aeronautics, and transportation. It is also leading in health care sectors and building oilfield equipment; only New York City is home to more Fortune 500 headquarters within its city limits. The Port of Houston ranks first in the United States in international waterborne tonnage handled and second in total cargo tonnage handled. Nicknamed the Space City, Houston is a global city, with strengths in business, international trade, entertainment, culture, media, fashion, science, sports, technology, education, medicine and research. The city has a population from various ethnic and religious backgrounds and a large and growing international community. Houston is the most diverse city in Texas and has been described as the most diverse in the United States. It is home to many cultural institutions and exhibits, which attract more than 7 million visitors a year to the Museum District. Houston has an active visual and performing arts scene in the Theater District and offers year-round resident companies in all major performing arts.

What part of Houston ranks first in the U.S. in international tonnage?
Port of Houston