From a bonfire lit by a fisherwoman to guide the menfolk to the shore, to the latest LED lights and a radon that can detect and guide ships, lighthouses have come a long way. These structures have played a crucial role in the growth of Chennai from a small fishing hamlet to a sprawling metropolis. The lighthouses at Chennai and Mamallapuram use dischargeable lamps, which rotate inside a bowl of mercury. Now, LED lights are preferred, say experts. According to Prasad, the Marina lighthouse maintains a range of 28 nautical miles for vessels and is one of the tallest lighthouses in the country.