Definitions of underground:

• noun:   a secret group organized to overthrow a government or occupation force
• noun:   electric underground railway
• adjective:   conducted with or marked by hidden aims or methods

  Example: "Underground resistance"

• adjective:   under the level of the ground

  Example: "Underground caverns"

• adjective:   used of independent armed resistance forces
• adverb:   beneath the surface of the earth

  Example: "Water flowing underground"

• adverb:   in or into hiding or secret operation

  Example: "The organization was driven underground"