ammonium sulphate solution ((NH4)2SO4) was developed. REEs-bearing soil was mined from orebodies and piled up on a flat leak-proof layer with a collecting dish at the bottom. (NH4)2SO4 solution was then injected on top of the soil and allowed for leaching. After 100 to 320 hours, REE extraction (with purity up to 90%) was collected for final processing. Due to stronger desorption capability of NH4+ compared with Na+, the technology had an improved final product quality and a reduction in lixiviant consumption. Hence, it had been used as a primary model of REE leaching process in the following 30 years. Modern mining methods (The third generation) In the last three decades, intense use of batch and heap leaching has posed a devastating and irreversible effect on the environment as well as the ecosystem in South China. Unregulated disposal of waste has also brought health problems to the residents near the mines. Thus, a compulsory in-situ leaching technology was implemented in 2011 to minimise aforementioned adverse effects. In-situ leaching technology requires comprehensive geological survey of local hydrogeological structure, rock joints pattern and ore characteristics in order to design a catchment area for the leaching process. Then, vertical leaching holes (0.8 m in diameter and 2 to 3 m apart) are drilled to reach the top of the REE-enriched layer (B) (1.5 to 3 m in depth) to allow injection of pressurised lixiviant (i.e. ~0.3M (NH4)2SO4). Finally, the REEs-loaded leaching solution is collected by recovery ponds at the bottom of orebody for final processing. Current research and development (Bioleaching) Recently, researchers have been developing various techniques to increase the yield of leaching REE. Bioleaching, a technique where REEs are solubilised by microbial activities or by-products of microbial metabolism, is actively studied as a greener alternative to the current method, which has been serious pollution to the environment. In terms of extraction effectiveness, some studies have reported that the recovery of REE by bioleaching could vary from less than 1% to nearly 90%. Thus, further understanding of the bioleaching mechanism is required before it is commercially practised. Applications of Rare Earth Elements Rare earth elements, the products of regolith-hosted REE deposits, are the fundamental building blocks of many daily-life high-tech products. Some of the examples and their applications are provided as follows. Neodymium is used in the production of strong magnets in loudspeakers and computer hardware with a smaller size and better performance.