The Great Smoky Mountains National Park is the most visited national park in the United States, and each year between eight and nine million people come to enjoy all the wonders it has to offer. The park covers area in both Tennessee and North Carolina and is about 16 miles wide by 50 miles long. Historic structures such as homes, mills and churches built by early settlers have been preserved and offer visitors a glimpse into the lives of those who lived here in the 1800s.   Dawn and dusk are the best times to spot some of the 70 kinds of mammals that call the park home, and bird lovers will be kept busy searching for the 235 species of birds that have been spotted thus far. The park is also known for having more varieties of fresh-water fish than any other national park in the country, and fishing is permitted year round with the appropriate license from Tennessee or North Carolina.   Perhaps the most famous inhabitants of the Park are the 400-600 black bears which visitors are encouraged to enjoy from a distance. About one-fourth of the park contains the trails that are paved or better known, and the majority of visitors stay on these; but many others venture out in search of the proverbial path less traveled. For these individuals there are more than 900 miles of well-marked hiking trails. For maximum safety, stay on the trails and away from waterfalls and cliffs. Please leave pets at home; they are not allowed on the trails. Yet another activity to enjoy in the Park is cross-country skiing. Old Newfound Gap and Clingman's Dome trails are best for this winter sport.   Please feel free to stop at either of the two visitor centers- Sugarlands on the Tennessee side or Oconaluftee on the North Carolina side. These centers offer information, books, pamphlets, and maps. Oconaluftee is directly adjacent to The Pioneer Farmstead that includes a meat house, chicken house, springhouse, blacksmith shop and a dwelling in a setting reminiscent of the 1800s.