Hattiesburg is a city in Forrest an Lamar Counties in the U.S. state of Mississippi. It is the principal city of the Hattiesburg, Mississippi Metropolitan Statistical Area which encompasses Forrest, Lamar and Perry counties. The MSA population exceeded 150,000 as a result of a 10% increase following Hurricane Katrina in August of 2005. It is Mississippi's fourth largest city and is known as "The Hub City". It has an incorporated suburb, Petal, and a presently unincorporated area, Oak Grove. It is the county seat of Forrest County, but the city has grown in recent years to include a portion of eastern Lamar County. Hattiesburg is home to The University of Southern Mississippi (originally known as Mississippi Normal College) and William Carey University (formerly William Carey College). Just south of Hattiesburg is Camp Shelby, the largest National Guard training base east of the Mississippi River.