Made using two synths. One is a noise generator that has been pitched down three octaves. It has a 12-hertz sine tremolo on either side, one of which is offset by half a wavelength. The other synth is a deep organ playing e3 and g3. I cut the sound in audacity, and the waves line up perfectly, so you don't need to worry about looping it badly.
Author: Rexen Devar
