Alpena is a city in the U.S. state of Michigan and the county seat of Alpena County. It is considered to be part of Northern Michigan. The Thunder Bay National Marine Sanctuary is located in the city. The population was 11,304 at the 2000 census. The population swells with a large number of visitors in the summer. Despite its small population, it is by far the largest city in the sparsely-populated Northeast Michigan area, serving as its commercial and cultural hub. It is considered to be one of the two anchor cities of Northern Michigan, along with Traverse City. The Alpena Regional Medical Center is a federally-designated rural regional medical referral center, and is the largest employer in the city.