The next revision (Block III) of the U.S. Virginia SSN (nuclear attack submarine) will contain an interesting modification. The twelve cruise missiles SSNs normally carried in individual compartments, will instead be carried in two "silos" that are about the same size as those used to carry sea-launched ballistic missiles in SSBN (ballistic missile nuclear subs). The technology for these two new silos in the Block III boats (which will first appear in the 11th ship of the class, in 2015) comes from that used in Ohio class SSGNs (cruise missile nuclear submarines). In the Ohios, each of the ballistic missile silos was converted to hold, and launch, seven Tomahawk cruise missiles. The Virginia silos will be a little smaller, and hold six cruise missiles each.