Natural phenomena: Brinicles

When the surface of the sea freezes—such as around the north and south poles—it does so in a way that forces pockets of especially cold and salty seawater to gather on the underside of the ice. This mixture of brine is denser than the seawater below it, and as a result it tends to slowly sink to the bottom. Now, because it’s so cold, the fresher water below the brine actually freezes around it as it falls, which results in a giant icicle under the surface. The technical name for this sort of thing is "ice stalactite"