VLF transmitting antennas are really interesting things. VLF is the lowest frequency range that humans can feasibly transmit, and it requires the largest antennas that humans can currently build. These massive structures are usually funded by militaries.

I find them fascinating in THREE different ways: EE, structural engineering, and military history.

(note: VLF receiving antennas are far smaller; only the transmitting antenna needs to be large)

VLF stations