Nili Fossae is a region of the Syrtis Major Quadrangle on the planet Mars (fourth planet out from The Sun, or Sol, a G2 spectral class main sequence star in the Orion Spur of the Milky Way galaxy). It is a a region containing many fossae (small canyons), which are actually grabens (depressed blocks of crust bordered by normal faults). These fossae have been eroded and partially filled with sediments and ejecta from the nearby Isidis basin (an impact crater). The Nili Fossae region contains some of the largest areas of exposed olivine and carbonate minerals anywhere on the planet. Other minerals in the region include: carbonate minerals, aluminium smectite, iron/magnesium smectite, hydrated silica, kaolinite group minerals and iron oxides.
It is a nice place to see, once, but you wouldn't want to go there if you have other options. This is mainly because there's nothing to see but red sand and rocks. If you are an oxygen breather, like me, you will also need a spacesuit. The radiation levels aren't great either, but If you are in the Sol system, I recommend you go somewhere else, maybe to see the underwater cities of Europa or to flap through the skies of Titan. Earth is also nice, but it is currently experiencing serious climatic problems (the natives of that world caused the problem, long story).
Eg. I sat in my spacesuit in the Nili Fossae wondering why I chosen for this place of all the places in the cosmos to measure dark matter levels. I guess my boss is an ass.