Hxen.com's tips：Every day persisted can learn English well!-->■Learn English Everyday■
vault – n. a locked room where money or valuable things are kept
sample(s) – n. a small amount of something that gives you information about the thing it was taken from
varieties – n. particular kinds of people or things
underground – adj. located or occurring below the surface of the earth
tunnel – n. a passage that goes under the ground or through a hill
construction – n. the act or process of building something
unit(s) – n. a machine or part of a machine or system that has a particular use
safeguard – v. to make someone or something safe or secure
crucial – adj. extremely important
insurance – n. protection from bad things that may happen in the future
