It looks like they just pumped water down the sides to loosen it up. Took ~1.5 hours or so in total.
Frames: 513
Time between frames: 12 seconds, playback 24fps
Camera: Sony A7 (mark I)
Lens: Samyang 35mm
Filmed at F7.1 100ISO with AEL. Exposure times from roughly 1/160 to 1/200