Known for its robust designs and large case sizes, Officine Panerai was established in 1860 by Giovanni Panerai in Florence, Italy. Around 1900, Giovanni's grandson Guido Panerai took over, and Panerai became the supplier to the Regia Marina — the Royal Italian Navy. After supplying the navy with high-precision instruments for a number of years, Officine Panerai created Radiomir, a radium-based powder that gives luminosity to the dials of sighting instruments and devices. By 1936, the Royal Italian Navy approached Panerai again with the request to design a watch resistant to extreme underwater conditions. The watch they created became known as the "Radiomir". Panerai's watches made during the early twentieth century era were comprised of cases designed and manufactured by Rolex SA, with Cortébert, a Swiss manufacturer, supplying the majority of their movements. The most recognizable designs from the firm are the Radiomir and Luminor. To date, vintage models from the first half of the 1900s, such as the reference 3646 and 6152 models, remain the most desirable among collectors.