This is the oldest section of the Białowieża National Park and covers an area of around 47.5 sq km, bordered to the north and west by the marshy Hwożna and Narewka Rivers, and to the east by the Belovezhskaya Pushcha National Park in Belarus. The terrain is mostly flat, swampy in parts, and covered with a mixed forest of oak, hornbeam, spruce and pine. Ancient trees reach spectacular sizes uncommon elsewhere, with spruce 50 m high and oak trunks 2 m in diameter; some of the oak trees are more than 500 years old. The forest is home to a variety of large mammals, including elk, roe deer, wild boar, lynx, wolf, beaver and bison. There are about 120 species of birds, including owls, cranes, storks, hazelhens and nine species of woodpecker. This part of the Park can only be entered in the company of an official guide, who can be hired through the PTTK Office or any one of several private tourist agencies. The mushroom collecting in this part of the Park is strictly forbidden.