Imagine you have a cave the ceiling of which is covered by a thin film of water. At t=0, there is an explosion which shakes the water and make it fall verticall

Imagine you have a cave the ceiling of which is covered by a thin film of water. At t=0, there is an explosion which shakes the water and make it fall vertically to the ground as droplets. You have a grid of water detectors on the grounds, which can tell you where and when a droplet hit the floor. If at t1 you detect a drop of water at floor coordinates (x,y), you know the height of the ceiling at (x,y) is h1 which you can compute from t1. So you can make an image of the ceiling. The paper algorithm is a fancier version of this, where the droplets are not falling straight down to the floor, but spread out like sphere ripples. So you need some fft math to deconvolve. The added benefit of using waves, is that you can see behind the objects, and you have a real 3D and not a 2.5D image.