Schumann Resonance

The Schumann resonance is a spectrum peak in the Earth's electromagnetic field spectrum, excited by lightning discharges in the upper atmosphere.

What it is
The resonance occurs when lightning disturbs ions in the atmosphere. The Earth's atmosphere just happens to be of the proper dimensions for electromagnetic radiation of extreme low frequency to resonate, which is exacerbated by the lightning strikes. Schumann resonances are the principal background in the electromagnetic spectrum, ranging from 3 to 60 Hz --on the threshold of human hearing.

Woo
The alleged mystery of the Schumann resonance has attracted New Agers and other woo-pushers to it.

Gregg Braden has claimed in his book Awakening to Zero Point that the resonance is increasing in frequency, and that this has something to do with the Fibonacci sequence. Of course, he misrepresents the harmonic frequencies in order to make this claim. In actual fact, the harmonic series of waves all occur at the same time, and can either reinforce each other or cancel out. If this implied that the Schumann resonance is increasing, then music would be impossible on string instruments.

Since the Schumann resonance covers about the same frequency range as brain waves, some New Agers go even farther, and assume that the Earth's ionospheric oscillations are evidence of the Earth's state of consciousness. And that therefore, programs which induce a different frequency of ionospheric oscillation (e.g. HAARP) are altering the Earth's consciousness, which will cause things like crop failures and droughts if you do it at a "bad" frequency. Conversely, there's also the claim that you can "heal the Earth" by inducing such oscillations at the frequency of alpha waves -- or that if enough people all put their own brains into an alpha-wave state at the same time, this will induce the Schumann resonance to oscillate at the same frequency and cause the same healing of Mother Earth.

In fiction
The New Age interpretation is mentioned in the anime series Serial Experiments Lain, in which it has something to do with the plot. We're not sure, really. But it still makes more sense than Braden's book.