Richard B. Dunn Solar Telescope

The Richard B. Dunn Solar Telescope is a unique vertical-axis solar telescope that is located at the National Solar Observatory at Sacramento Peak. The optical path starts at a heliostat on top of a 136-foot-tall tower and continues 193 feet more underground to the primary mirror. It then returns to one of six quartz optical windows in the floor of an optical laboratory at ground level.
-from Wikipedia