The crossword solution for the term 'Sink in the Middle' is SAG, which stands for Subsidence of an Area of Ground. This is a geological term that describes the gradual downward movement of the Earth's surface due to natural causes such as erosion or compaction of soil. It can also be caused by human activities such as mining or construction. SAG is often used to describe the sinking of land in areas with high water tables or areas prone to flooding.