Later studies have found that olivine-rich rocks cover more than 113,000 square kilometers of the Martian surface. That is 11 times larger than the five volcanoes on the Big Island of Hawaii. On December 6, 2006 NASA released photos of two craters called Terra Sirenum and Centauri Montes which appear to show the presence of liquid water on Mars at some point between 1999 and 2001. Hundreds of gullies have been discovered that were formed from liquid water, possible in recent times. These gullies occur on steep slopes and mostly in certain bands of latitude. Below are some examples of gullies that were photographed by Mars Global Surveyor. A few channels on Mars displayed inner channels that suggest sustained fluid flows. The best-known is the one in Nanedi Valles. Another was found in Nirgal Vallis. Many places on Mars show dark streaks on steep slopes, such as crater walls. Dark slope streaks have been studied since the Mariner and Viking missions. It seems that streaks start out being dark, then they become lighter with age. Often they originate with a small narrow spot, then widen and extend downhill for hundreds of meters. Streaks do not seem to be associated with any particular layer of material because they do not always start at a common level along a slope. Although many of the streaks appear very dark, they are only 10% or less darker than the surrounding surface. Mars Global Surveyor found that new streaks have formed in less than one year on Mars. Several ideas have been advanced to explain the streaks. Some involve water, or even the growth of organisms. The generally accepted explanation for the streaks is that they are formed from the avalanching of a thin layer of bright dust that is covering a darker surface. Bright dust settles on all Martian surfaces after a period of time. Dark streaks can be seen in the images below, as seen from Mars Global Surveyor. Some parts of Mars show inverted relief. This occurs when materials are deposited on the floor of a stream then become resistant to erosion, perhaps by cementation. Later the area may be buried. Eventually erosion removes the covering layer. The former streams become visible since they are resistant to erosion. Mars Global Surveyor found several examples of this process. Many inverted streams have been discovered in various regions of mars, especially in the Medusae Fossae Formation, Miyamoto Crater, and the Juventae Plateau. The image below shows one example. Pathfinder found temperatures varied on a diurnal cycle. It was coldest just before sunrise (about −78 Celsius) and warmest just after Mars noon (about −8 Celsius).