The geology of the Bangkok area is characterized by a top layer of soft marine clay, known as "Bangkok clay", averaging  in thickness, which overlies an aquifer system consisting of eight known units. This feature has contributed to the effects of subsidence caused by extensive ground water pumping. First recognized in the 1970s, subsidence soon became a critical issue, reaching a rate of  per year in 1981. Ground water management and mitigation measures have since lessened the severity of the situation, and the rate of subsidence decreased to  per year in the early 2000s, though parts of the city are now  below sea level. 