Nili Fossae is a group of large, concentric grabens on Mars, located in the Syrtis Major quadrangle. Nili Fossae was on the list of potential landing sites of the Mars Science Laboratory, arriving in 2012, but was dropped before the final four sites were determined. In September 2015, it was selected as a potential landing site for the Mars 2020 rover, which will use the same design as Curiosity, but with a different payload focused on astrobiology. (wikipedia entry)
A fossa is a long, narrow, shallow depression. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.