Discernible in this north-looking, low-oblique photograph are numerous transportation arteries that radiate from metropolitan Houston, the fourth most populous city in the United States. Houston is a great financial, industrial, commercial, and medical hub and one of the world’s major oil centers. Extending generally eastward from the city center is the snakelike Houston Ship Channel, along which is located one of the world’s greatest concentrations of petrochemical and oil refinery industries. The Port of Houston is the third-busiest port in the United States. Metropolitan Houston, with numerous active faults crisscrossing the city, rests on the United States’ third largest aquifer and the flat surface of the gulf coastal plain, a giant wedge of mud and sand deposited into the Gulf of Mexico during the last ice age by ancestral Texas rivers. Visible are the runways of George Bush Intercontinental Airport north of the city; sediment-laden Lake Houston to the east; and a small portion of Galveston Bay to the south.