The lowest possible temperature is 0 Kelvin (-459.67 °F, -273.15 °C), abbreviated 0 K. While attaining exactly 0 K is thermodynamically impossible, extremely low temperatures have been achieved in the lab using a combination of laser cooling and evaporative cooling.

MIT holds the current record, 450 pK (petaKelvins), or 4.5 × 10^-10 K, which was announced in September 2003. The Helsinki University of Technology's Low Temperature Lab achieved a nuclear spin temperature of 100 pK, but this pertains only to the nuclear spin and not all thermodynamic degrees of freedom.

Science has been engaged in subzero temperature research primarily since the advent of modern refrigeration in the mid-19th century. It is thought that about 800 million years ago, the majority of the planet was covered in subzero temperatures, giving rise to a planet-wide icecap sometimes called "Snowball Earth". As recently as 10,000 years ago, a planetary Ice Age was still in effect, known as the Wisconsin glaciation. This Ice Age may have prevented the rise of civilization. Its passing was followed by the Neolithic revolution and modern agriculture.

Here are some subzero temperatures and their relevance:

• 450 pK - lowest temperature achieved, by MIT, in 2003.
• 170 nK - first temperature at which a Bose-Einstein condensate, a unique state of matter where a group of atoms collapses into the lowest quantum state of the external potential, allowing quantum effects to be observed on the macroscopic level.
• 700 nK - former record low lab temperature, by NIST, in 1994. >li>0.95 K - melting point of helium.
• 1 K - coldest known region of outer space, the Boomerang nebula.
• 2.17 K - temperature below which helium is in a superfluid state, demonstrating bizzare properties like crawling up out of a jar it is placed in.
• 2.7 K - average temperature of outer space, the thermal echo of the Big Bang.
• 3 K - approximate temperature of typical liquid helium.
• 4.1 K - temperature at which superconductivity was first demonstrated, using mercury.
• 65 K - approximate temperature of liquid nitrogen.
• 123 K - informal boundary between cryogenics and refrigeration.
• 150 K - temperature of the hottest known superconductor, the compound SnBa₄Tm₄Cu₆O₁₈.
• 183.7 K - coldest terrestrial temperature ever recorded, at Vostok, Antarctica.
• 273.15 K - melting point of water.

Subzero temperature research is an important part of science. Without knowledge of the subzero regime, we would know a lot less about physics and chemistry, not to mention that we'd have to eat our food quite quickly, before it began to rot from lack of refrigeration!