YouTuber KittyPouncer created this terrific timelapse video of curly icicles extruding from pipes. Here's how it happens:
Water, expanding as it freezes, squeezes pre-existing ice out of holes. After reading the invaluable blog linked below, I wanted to try this myself, but to film it happening as a time-lapse. I modified the method in the blog slightly, for convenience. I used 1/2-inch galvanized steel or black steel pipe, based on price and availability. I bought a 5-inch piece and a 4-foot piece, a connector, and two end caps. I cut a slot in one cap with a wide blade, and later added a small home. I screwed this onto the short pipe, sealed the holes with plastic wrap and rubber band, filled it with water, and froze it in my freezer. I then chilled a container of water outdoors until it had ice on top. At night, once the outdoor temperature was below -4C, I removed the plastic from the small pipe, screwed it to the large pipe (using masking tape to seal the threads), filled the large pipe with the cold water, and screwed on the cap. I laid the pipe almost horizontal, with the ice end lower. As the water froze, it expanded. With nowhere else to go, it squeezed the ice through the holes, making the patterns seen here. A couple times I divided the slot in two with a nail. I also tried horizontal and vertical layers of coloured water. I was going to go back and get more caps and try different holes, and try flattening the pipe instead of using a cap, but didn't get back to Home Depot. It was March when I started, and there weren't that many cold nights. The nights were also getting shorter, so that sunrise happened before the experiments were over, messing up my lighting. Temperatures of -7 or so worked best. -12 seemed to make the ice more brittle, and it broke off as it extruded. At -4 it didn't freeze at all before morning. Perhaps the pressure lowered the freezing point. I don't know why some extrusions were smooth and clear, and others were jagged and white.