Packed Ice

"Ice is also a rather charming-looking building block, but its aesthetic properties are rather undercut by its tendency to melt. Locate the rare Ice Spikes biome, however, and you can find packed ice, a variety of the chillsome block which doesn’t turn into a puddle under the glare of light sources. Be careful mining it though, as only a pick-axe with the Silk Touch enchantment will harvest it without shattering the block into nothingy nothingness."

- Marsh Davies

Packed ice is a solid block variant of ice.

Obtaining
Packed ice can be obtained using any tool enchanted with Silk Touch. Without Silk Touch, breaking it drops nothing.

Natural generation
Packed ice is found in the rare Ice Spikes biome.

Packed ice generates as igloo windows.

Packed ice also generates as part of icebergs.

Crafting
Packed ice is slippery, causing most entities to slide, including items. This also allows for increased speed of items in water currents by placing the packed ice under the water current. Mobs do not travel faster in water currents on packed ice.

When a non-full block is placed on top of packed ice, the block has the same 'slipperiness' as the ice below it; although, if packed ice is placed below soul sand, it amplifies the slowing effect of the soul sand rather than making it slippery.

Mobs can spawn on packed ice, unlike normal ice. This can allow for compact mob farms that rely on the tendency of pushed mobs to slide off the ice in some way.

Packed ice is a completely solid block, and allows placement of any objects on top.

Unlike normal ice, packed ice will not melt if placed near light sources.