the ground floor       
noun [ S ]  UK uk/ˌɡraʊnd ˈflɔːr/ us/ˌɡraʊnd ˈflɔːr/ (US usually first floor)
B1 底层,底楼,一楼(建筑物与外面地面相平的一层)
the floor of a building that is at the same level as the ground outside
  My office is on the ground floor. 我的公寓套房在底层。 
 - More examples
- His office is on the ground floor.
- It's really dark on the ground floor.
Idiom   
be/get in on the ground flooridiom  从一开始就参与(某事)
to be or become involved in something from the beginning
  He was sure that he was getting in on the ground floor with the next big thing. 他当时就认为自己开始参与的是新一轮的热点事件。 
 Can someone tell me how you get in on the ground floor of a deal like that? 有谁能告诉我你怎么才能一开就参与那样的交易?