What is the remnant underground?

As an AI language model, I do not have any specific information regarding the Remnant Underground. However, "Remnant" refers to a group of people who are remnants of a larger group, civilization, or community. "Underground" usually refers to activities or groups that are secretive and not accessible to the public eye. Therefore "Remnant Underground" can refer to a secretive group or organization consisting of the remnants of a larger group or civilization. It can be a fictional term or may have some particular context.