Scientists in Singapore and China say it is due to a hidden force – a coulomb repulsion between the unevenly-bound bonding and non-bonding electron pairs in the hydrogen bond. Understanding the behaviour of water is key to many fields of science and engineering.

Link to journal article
The hidden force opposing ice compression
C Q Sun, X Zhang and W Zheng
Chem. Sci., 2012, DOI: 10.1039/c2sc20066j