The SMB Mine. In 1937 the SMB mine was in development at low priority for use in depths of 6,750 to 10,000 feet. It was designed to use a large steel case, and it was to employ a new-type anchor to be designed. The SMB mine case was of the same dia-meter as the SMA, but was approximately double the length. It was to be laid by the same type submarine as the SMA mine.