The Museu Nacional Honestino Guimarães is a windowless white concrete dome 50 metres in diameter, half-buried in the Esplanada dos Ministérios. Designed by Niemeyer late in his career (it opened in 2006), it pairs visually with the National Library directly opposite — a long horizontal box facing a sphere.
Inside, two suspended ramps and a wide central hall host changing temporary exhibitions, often contemporary art or design. The space itself, however, is the real attraction. Free entry; check the listings for what's on.