1.a secret group organized to overthrow a government or occupation force
2.electric underground railway
3.under the level of the ground;
belowground storage areas
4.conducted with or marked by hidden aims or methods;
clandestine intelligence operations
5.used of independent armed resistance forces;
guerrilla warfare
6.in or into hiding or secret operation;
the organization was driven underground
7.beneath the surface of the earth;
water flowing underground