The sponge is the interior of a comet and the cavities are on the sun. At https://phys.org/news/2020-09-presence-resonating-cavities-sunspots.html … sun spots are darker regions that appear periodically on the face of the sun. They are caused by strong concentrations of a magnetic field, it is thought. Oscillations in the sun's atmosphere has been interpreted as magnetic waves. Sun spots appear to transfer energy from interior layers to the outer regions of the sun's atmosphere. New results show these magnetic waves are trapped in a region of the atmosphere above the sun spots, giving rise to resonances. The waves are confined in what looks like a cavity, strengthening certain frequencies. Numerical simulation on a super computer was involved.
At https://phys.org/news/2020-09-rainbow-comet-heart-sponge.html … a rainbow comet with a heart that is sponge like. Hopefully, not as soft as a Victoria sponge with plenty of cream, but as in a sponge that conforms to the mainstream model of comets as being a fluffy conglomerate. The rainbow colours are explained at the link. New analysis suggests the interior of the comet visited by ESAs Rosetta Mission, a few years back, is porous – quite unlke the hard as rock exterior. Like a marshmellow in a brittle chocolate shell, one might say, or a sponge still soft on the inside but stiff with dried soap on the outside. Mainstream theory still standing it seems. They are thinking in terms of the interior of this comet, if not other comets as well, as pristine, an interior that may give clues on the formation of the solar system.