The highest sustained temperature mankind can produce is achieved in experimental fusion reactors, such as those in the UK, where plasma is contained using magnetic fields in a toroidal shape. This method allows for temperatures around 10^8 K, similar to those found in the Sun's core. Thermonuclear devices, like bombs, can reach similar temperatures but are not sustained. The discussion highlights the importance of containment methods for achieving and maintaining high temperatures. Overall, the focus is on the advancements in fusion technology for producing extreme temperatures.