With an estimated 1.6 billion tonnes of water ice at its poles and an abundance of rare-earth elements hidden below its （A） s ， the Moon is rich ground for mining. In this month's issue of Physics World， science writer Richard Corfield explains how private （1） c and space agencies are dreaming of tapping into these lucrative（有利可图的）resources and turning the Moon's grey landscape into a money-making conveyer belt.

Since NASA disbanded its manned Apollo missions to the Moon over 40 years ago， unmanned （B） s has made giant strides and has identified a rich supply of water ice at the north and south poles of the Moon. "It is this， more than （2） a else，" Cornfield writes， "that has aroused interest in mining the Moon， for where there is ice， there is fuel."

Texas-based Shackleton Energy Company （SEC） plans to mine the vast reserves of water ice and convert it into rocket propellant in the form of （C） h and oxygen， which would then be sold to space partners in low Earth orbit.

As the company's chief executive officer， Dale Tietz， explains， the plan is to build a "gas station in space" in which rocket propellant will be sold at prices significantly lower than the cost of sending （D） f from Earth.

SEC plans to extract the water ice by sending humans and （E） r to mine the lunar poles， and then use some of the converted products to power mining hoppers， lunar rovers（月球车）and life support for its own activities.

As for mining the rare-earth （3） e on the Moon， China is making the most noticeable headway. The Jade Rabbit lander （玉兔着陆器）successfully touched down on the Moon in December 2013 and the Chinese space agency has publicly suggested establishing a "base on the Moon as we did in the South Pole and the North Pole." As rare-earth elements are vital for everything from mobile phones to （F） c and car batteries， it is no surprise that China may want to cast its net wider.

"All interested parties agree that the Moon – only one step from （4） E - is the essential first toehold（立足点）for humankind's march to the stars." Corfield concludes.

（A， B， C， D， E， F FOR CROSS， 1， 2， 3， 4 FOR DOWN. The first letters of the absents were given）