An underground transmission line is a high-voltage electrical cable system buried beneath the surface, designed to move large amounts of electricity over long distances. The key difference lies in the voltage and purpose:

• transmission lines operate at very high voltages (e.g., 132kV, 230kV, 500kV) to transport bulk power from generation sources (like power plants or wind farms) to substations in large population centers. They are the "highways" of the electrical grid.

• Distribution Lines operate at lower voltages (e.g., 1kV to 35kV) to distribute power from substations to individual homes, businesses, and buildings. They are the "local roads" of the grid.

The installation of underground transmission lines is significantly more complex and expensive due to the cable's immense size, weight, and the stringent technical requirements needed to manage immense heat and electrical stress.