The Giant Magellan Telescope (GMT) is one of the most ambitious telescope projects in the world, and its mirrors are a critical part of its design. The GMT will have seven 8.4-meter (27.5-foot) mirrors, each of which will be made of a very thin layer of glass with a reflective coating. The mirrors will be made by melting a 36,287 kilogram (80,000-pound) block of borosilicate glass and casting it with precision. Casting a single GMT mirror takes almost 18 months. The finished mirror will be transported 240 kilometres (150 miles) from Arizona, through the city of Tucson, before being craned up to the top of the telescope's main enclosure, some 3.6 kilometres (12,000 feet) above sea level.

Here are the steps involved in building a mirror for the GMT:

1. Select the right glass. The glass for the GMT mirrors is made from a special type of borosilicate glass that has a very low coefficient of thermal expansion. This means that it will not change shape or size significantly with changes in temperature. This low thermal expansion is critical because the mirrors must be able to hold their shape precisely, even when they are exposed to the changing temperatures of the night sky.

2. Melt the glass. The first step in making the mirror is to melt the glass. This is done in a special furnace that can heat the glass to a temperature of over 1,500 degrees Celsius (2,700 degrees Fahrenheit). The melting process takes about 24 hours.

3. Cast the glass. Once the glass is melted, it is poured into a mold. The mold is made of iron and has the same shape as the desired mirror. The glass is allowed to cool and solidify in the mold. The casting process takes about two days.

4. Grind and polish the glass. After the glass has cooled, it is removed from the mold. It is then ground and polished to create a smooth surface. This process is done using a variety of tools, including diamond saws, grinding wheels, and polishing pads. The grinding and polishing process can take several weeks.

5. Coat the glass with a reflective layer. The final step in making the mirror is to coat it with a reflective layer. This is done using a process called vacuum deposition. In this process, a thin layer of aluminum or silver is deposited on the glass surface in a vacuum chamber. The reflective coating is about 100 nanometers thick.

Once they are completed, the mirrors will be transported to the Las Campanas Observatory in Chile, where the GMT is being built. The mirrors will be installed on the telescope's main structure, which is expected to be completed by the early 2030s.