What is the meaning of “significant and substantial”?

Under the case law decided by the Commission, four elements must be proven to establish a "significant and substantial" (S&S) violation: 

1. The underlying violation of a mandatory standard. 
2. The existence of a discrete safety hazard contributed to by the violation. 
3. A reasonable likelihood that the hazard contributed to or will result in an injury.
4. A reasonable likelihood that the injury in question will be of a reasonably serious nature. 

Secretary of Labor v. Mathies Coal Company, 6 FMSHRC 1 (January 1984)