Pyramid of Dahshur
This area is arguably the most important pyramid field in Egypt outside Giza and Saqqara, although until 1996 the site was inaccessible due to its location within a military base, and hence was virtually unknown outside archaeological circles.
The southern Pyramid of Sneferu, commonly known as the Bent Pyramid is believed to be the first (or by some accounts, second) attempt at creating a pyramid with smooth sides. In this it was only a partial — but nonetheless visually arresting — success; it remains the only Egyptian pyramid to retain a significant proportion of its original limestone casing, and serves as the best example of the luminous appearance common to all pyramids in their original state.
The northern, or Red Pyramid built at the same location by Sneferu was later successfully completed as the world’s first true smooth-sided pyramid. Despite its relative obscurity, the Red Pyramid is actually the third largest pyramid in Egypt — after the pyramids of Khufu and Khafre at Giza. Also at Dahshur is the pyramid known as the Black Pyramid of Amenemhet III.
Source: http://en.wikipedia.org/wiki/Egyptian_pyramids