It’s one of the most fundamental compounds on Earth, and it makes up roughly 60 percent of the human body, and yet water is turning out to be stranger than we could have ever imagined.

Researchers have been investigating the physical properties of water, and found that when it’s heated to between 40 and 60 degrees Celsius, it hits a 'crossover temperature', and appears to start switching between two different states of liquid.

As a chemical compound, water is so vital to life on Earth, we’ve been underestimating how legitimately weird it is.

We’ve all gotten so used to it, it’s hard to imagine things getting any more complex than the three basic states: solid, liquid, gas. (Under very rare circumstances, a plasma-like state can also form.)

But in many ways, plain, old water is unlike any other substance on the planet.

With the exception of Mercury, water has the highest surface tension of all liquids. It’s also one of the only known substances whose solid state can float on its liquid state, and unlike almost every other known substance, water expands when it freezes.

It also has a bizarre boiling point. While the boiling points of other hydrides, such as hydrogen telluride and hydrogen sulphide, decrease as their molecule size decreases, H2O has a surprisingly large boiling point for such a small molecular weight.

"No one really understands water," Philip Ball points out in Nature. "It’s embarrassing to admit it, but the stuff that covers two-thirds of our planet is still a mystery. Worse, the more we look, the more the problems accumulate: new techniques probing deeper into the molecular architecture of liquid water are throwing up more puzzles."

Now physicists have demonstrated that somewhere between the temperatures of 40 and 60 degrees Celsius (104 and 140 degrees Fahrenheit), liquid water can 'switch' states, exhibiting a whole new set of properties depending on the state it flips to.

To figure this out, an international team led by physicist Laura Maestro from the University of Oxford in the UK looked at a number of specific properties of water.

They looked at things like thermal conductivity, refractive index, conductivity, surface tension, and the dielectric constant - how well an electric field can spread through a substance - and how they responded to fluctuations in temperature between 0 and 100 degrees Celsius.

Once the water hit 40 degrees, things started to shift, and properties were changing all the way up to 60 degrees. Each property had a different 'crossover temperature' somewhere within this threshold, and the researchers suggest that this is because the liquid water had switched into a different phase.

The team lists a few of these crossover temperatures: approximately 64 degrees Celsius for thermal conductivity, 50 degrees Celsius for refractive index, about 53 degrees Celsius for conductivity, and 57 degrees Celsius for surface tension.

"These results confirm that in the 0-100 degrees Celsius range, liquid water presents a crossover temperature in many of its properties close to 50 degrees Celsius," they conclude.

So what's going on here? It's not yet clear, but the fact that water could be switching between two entirely different states of liquid at certain temperatures could be linked to why H2O has such unusual properties in general.

Water molecules maintain only very short-lived connections between each other, and these hydrogen bonds are actually far weaker than the bonds that link the individual hydrogen and oxygen atoms inside the molecules.

For this reason, the hydrogen bonds that link water molecules together are constantly breaking and reforming, and yet within all that chaos, set structures and 'rules' persist. Physicists suspect that this is what gives water its unusual properties - but no one's entirely sure how it works.

"Everyone is agreed that one aspect of water’s molecular structure sets it apart from most other liquids: fleeting hydrogen bonds," Ball writes for Nature.

"These feeble bonds that link the molecules constantly break and form above water’s melting point, yet still impose a degree of structure on the molecular jumble. That’s where the consensus ends."

While Maestro and her team's results will need to be replicated by an independent team before we can start rewriting textbooks to reflect the four (or 3.5?) states of water that could potentially exist, they say their discovery could have big implications for our understanding of both nano and biological systems.

"For example, the optical properties of metallic (gold and silver) nanoparticles dispersed in water, used as nanoprobes, and the emission properties of ... quantum dots, used for fluorescence bioimaging and tumour targeting, show a singular behaviour in this temperature range," they write in their paper.

"[It also] raises the question of whether temperature-driven structural changes in water affect biological macromolecules in aqueous solutions, and in particular in proteins, which are the vital functional biological units in living cells."

The research has been published in the International Journal of Nanotechnology.