In the center,at ground level, is a round pool of water.

VOA: special.2009.05.25

So if we go to the ground state, what you see is we're at that lowest energy level, and we only have one possibility for an orbital, because when n equals 1, that's all we can do.

如果我们在基态上，你可以看到，我们在能量最低的态上，只有一种，可能的轨道，因为n等于1，只有这种可能。

麻省理工公开课 - 化学原理课程节选

Later,we visit rooms below ground level.

VOA: special.2009.11.16

And the fields are commonly divided between level ground and a mountainside.

VOA: special.2010.04.13

WMO scientists assess and monitor air pollutants such as ground-level ozone,smog, particulate matter, sulfur dioxide and carbon monoxide.

VOA: standard.2009.03.23