The Sambadrome is a 700-metre avenue with permanent bleachers on both sides, designed by Oscar Niemeyer and opened in 1984. It is the venue for the main samba school processions during Rio Carnival in February — the competition between the elite samba schools that is simultaneously a sporting contest, a theatrical spectacle, and an outpouring of creative energy without equal anywhere. Each school brings 3,000 to 5,000 participants, weeks of work on elaborate floats, and the kind of organisation that would be impressive in a military campaign.
Outside Carnival season, the Sambadrome is used for concerts and special events, and you can walk the length of the avenue freely. The rehearsals of the samba schools (quadra sessions) begin in October and November and are open to the public — attending one of these is a better introduction to Carnival culture than the main event itself, which can be expensive and overwhelming. Check the websites of the main schools (Mangueira, Portela, Beija-Flor) for their rehearsal schedules.