Negative space is art not physics

 I was drawn to this recent art installation at Boughton House in Northamptonshire.

It stands opposite a mound made of the earth dug out centuries ago to make the ornamental water features. The mound has the same dimensions up as the hole has down.

The interpretation board mentions it as a "negative space". Now if I were taking space to be a volume, I'd say it was a scalar and could not be negative. So for example with Charles' Law for gases, I plot a line for volume increasing with temperature and then extrapolate backwards to zero volume. That temperature is called absolute zero because you cannot go any lower and have negative volume. I was wondering whether perhaps there might be negative space in General Relativity but I don't think there is. However, it turns out that negative space is a term from art.