Natural background noise increases as frequency decreases, so a lot of radiated power is required to overcome it. Worse, small antennas (relative to a wavelength) are inherently inefficient. This implies high transmitter powers and very large antennas covering square kilometers. This precludes submarines from transmitting VLF, but a relatively simple antenna (usually a long trailing wire) will suffice for reception. I.e., VLF is always one-way, from land to boat. If two-way communication is needed, the boat must ascend nearer to the surface and raise an antenna mast to communicate on higher frequencies, usually HF and above.