Water

Usually water is defined as the Negation of Islands.

KURŠ, Jan, LUNGU, Mircea, IYADURAI, Rathesan and NIERSTRASZ, Oscar, 2015. Bounded Seas. Computer languages, systems & structures. 2015. Vol. 44, p. 114–140. DOI https://doi.org/10.1016/j.cl.2015.08.002.