Hydrogen bonding between water molecules. H bonding is responsible for many of the important properties of water such as high cohesion, surface tension, boiling point and its role as a universal solvent.
This sea of thousand year old ice is so blue from having the oxygen squeezed out of it. This causes the ice crystals to become larger, which in turn means that more of the oxygen-hydrogen stretch bond is visible to the human eye. The oxygen-hydrogen bond absorbs light at the red end of the spectrum, which is why we see the blue so clearly.