Nuclear quantum effect of protons in terms of tunneling and delocalization plays an important role in many physical and chemical properties of water.In spite of tremendous theoretical efforts,the direct experimental verifcation of the nuclear quantum effect in water is still lacking.Ideally,attacking this problem requires the access to the internal degrees of freedom of water molecules,which remains a great challenge owing to the light mass and small size of hydrogen.Using a low-temperature scanning tunneling microscope(STM),we are able to resolve in real space the internal structure,that is,the O-H directionality,of water molecules adsorbed on solid surfaces [1,2].The key steps are decoupling electronically the water from the metal substrate by inserting an insulating NaCl layer and tuning controllably the molecular density of states of water around the Fermi level via tip-water coupling.These techniques allow us not only to visualize the quantum tunneling of protons within the H-bonded network,but also to quantify the effect of proton delocalization on the H-bonding strength at the single bond limit.