Here's why:

* Wavelength and frequency are inversely proportional. This means that as the wavelength of a signal increases, its frequency decreases.

* ELF waves have frequencies ranging from 3 Hz to 30 Hz. This makes them extremely low frequency compared to other types of electromagnetic radiation.

* Long wavelengths correspond to low frequencies. As a result, ELF waves have the longest wavelengths among all types of electromagnetic radiation.

For example, visible light has a much higher frequency than ELF waves, and therefore its wavelength is much shorter.