Do research in gases. While completing his doctorate work at Groningen in the 1870's, Onnes began to study with Diderik van der Waals. It was at this time, and under van der Waals' mentorship, that Onnes' interest in locating a "mythical perfect gas" started to peak. In part, this interest led him to begin studying liquid hydrogen.
Focus on liquid hydrogen. While at Leiden University in the 1880's, Onnes sought to expand on van der Waals' scientific inquiry and work with gases. He started exposing gaseous elements to harsh temperatures, particularly extreme low temperatures. This led him to set up a cryogenic laboratory where, by 1906, he and his team were producing liquid hydrogen in considerable quantities.
Begin studying mercury. After progressing from his work with hydrogen to helium, Onnes deliberated on whether his freezing techniques could be applied to metals. The fact that the resistivity of metals depended on temperature made Onnes believe that he might be able to lower or eradicate that resistivity in mercury if the mercury was cold enough.
Freeze liquid mercury. Onnes was able to lower the temperature of a small batch of pure mercury to 4.2 degrees Kelvin (-452.11 degrees Fahrenheit). He observed that electrical resistance in the cooled mercury completely disappeared. Thus, the first superconductor was found and the principle of superconductivity established.