Mare Tranquilitatis Hole - a habitable place for a first lunar settlement

Space stations on the lunar surface are exposed to cosmic rays, solar flares, micrometeorites and huge temperature variations. Therefore human outposts on the lunar surface have to be covered by huge layers of regolith. In 2009 the Japanese lunar orbiter SELENE (Selenological and Engineering Explorer) has detected three giant lunar holes: Mare Tranquilitatis Hole (MTH), Marius Hills Hole and Mare Ingenii Hole. The holes differ from normal impact craters and may be the entrances to underground lave tubes. The deepest one is MTH with 107 m and 98 x 84 m in diameter. According to Haruyama et al. the soil of lunar holes could contain water resources (protons from solar-wind hydrogen flux or even water molecules). Lunar holes reduce the effects of cosmic rays because of their limited field of view from the bottom. They provide also milder temperatures than the lunar surface. In the shadowed areas the temperature  ranges from -20°C to +30°C during the lunar day. These benefits make lunar holes become favourite locations to establish initial lunar stations. In a first step we propose to build an initial base on the lunar surface at the edge of MTH. It can be used for storage and as a "site hut" for astronauts to supervise the following work. In a second step the initial base is enlarged by a modular structure down to the bottom of MTH. Robotic and semi-robotic machinery is used to erect the modular structures. Lunar regolith is used for protection against cosmic rays and meteorites (ISRU In Situ Resource Utilization). Finally MTH could be  closed by a transparent dome and filled with air to create a "green" habitat for human settlers.