Northwest of Salvador and south of Alter do Chao, the 2,000 square mile Tapajos National Forest is the largest rainforest in the world. There are tons of hiking options, but watch out for jaguars.
The Tapajos National Forest is the home of the annual Jungle Marathon, a 200km multi-day adventure race.
To reach Tapajos National Forest, it is best to fly into the Santarem Airport via TAM Airlines
When traveling to Brazil, there are two main international airports: Galeao International Airport in Rio de Janeiro and Guarinos International Airport in San Paulo. Since it is a large country and you may wish to travel domestically by air, consider coming in to the country via TAM Airlines and purchasing a heavily discounted Brazil Air Pass which connects with domesitc airports. Note: there is a hefty tourist visa which was $100 in 2007.
Overall, Tapajos National Forest is the 1st most popular hiking trail of all 2 hikes in Brazil.