Scientists have long puzzled over this particular form of magnesium carbonate since it doesn't normally occur in nature and has defied synthesis in laboratories. Until now, its properties have remained a mystery. 

Today I learned the word ADSORB:

Called upsalite in honor of the university where it was discovered, the material features a surface area of 800 square meters per gram. It's got the highest surface area measured for a synthesized alkali metal carbonate. And in addition, upsalite is filled with empty pores all having a diameter smaller than 10 nanometers.

This means that it can absorb — or more accurately, adsorb — more water at low relative humidities than the most advanced materials currently in existence.

Unlike absorption, where fluids permeate or are dissolved by a liquid or solid, adsorption involves the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. And it does so as a consequence of surface energy (similar to surface tension).