In the 18th century, tajchy gradually expanded into a complicated system of lakes and channels, designed by three famous engineers: Jozef Karol Hell, his father Maximilian Hell, and especially Jozef's teacher Samuel Mikovíny. The project was technically and economically demanding. For example, construction of two Richňava tajchy required the work of 4,000 people working every day for three years (1738–1740). But after their completion, tajchy proved capable of draining all the water from the flooded mines and they even provided enough energy for other booming industries.