Llanesco is a Martian impact crater, 27 kilometers in diameter. According to a surface age map of Mars based on US Geological Survey data, the area around Llanesco is from the Noachian or Hesperian epoch, which places the area's age at 3.8 to 1.8 billion years. The crater's rim averages about 7,750 meters above zero altitude, and its floor averages about 7,000 meters above zero altitude. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On Mars large craters are named after deceased scientists, writers and others who have contributed to the lore of Mars. Small creaters (less than 60km) are named after small towns and villages.