In the early 2000s, Amsterdam locals frustrated with sound pollution demanded the city do something to address it. The low frequencies and long wavelengths worked against traditional barrier solutions, but airport staff realized that plowed fields in the area seemed to dampen the noise — especially ones with a particular spacing of ridges.
In 1998, political leaders in Northern Ireland signed a peace agreement. They hoped it would end the sectarian violence between Catholics and Protestants who'd been fighting each other for decades. At the root of the conflict was the question of whether Northern Ireland should reunite with the rest of Ireland — or whether it should remain part of the United Kingdom.