Singing sand

Singing Sand is a special kind of sediment that creates squeaking-like noises when walked on, so as to make it easier to detect intruders, due to its high-quartz concentration.

Operation Intrude N313
Solid Snake encounters this sand on his way to destroy the Hind D. According to Holly White, the sand was imported from Okinawa, Japan, so one can assume that its position as a noisemaker was intentional. However, crawling over it makes no noise. Zanzibarland Mercenaries have special boots that doesn't allow the sand to squeak when stepped on.

Trivia
- Singing Sand really does exist. It is also known as whistling sand, and barking sand.

-Singing Sand dunes exist all over the world, although they are rare in Okinawa nowadays, because Singing Sand loses its "voice" from pollution, so dunes in Japan are mostly now mute. The Sand in ZanzibarLand is immune to crawling and wind, so it might be a sign of pollution.