Iranian media has shown footage of a submerged US Navy submarine allegedly taken in the vicinity an Iranian Navy training exercise near the Strait of Hormuz. The footage, shared on social media by Twitter user Aurora Intel, was taken from an Iranian Navy SH-3 Sea King helicopter:
It appears to show an Ohio Class cruise missile submarine (SSGN). The location and timing makes this almost certainly the USS Georgia (SSGN 729}.
The USS Georgia transited into the Persian Gulf via the Strait of Hormuz on December 21, escorted by US Navy cruisers and patrol boats. The inbound transit was made on the surface.
The circumstances of the shallow pass are unclear. The submarine is shown at periscope depth. A single DDS (dry deck shelter) is seen on the port side of the casing behind the sail. This hangar can carry SEAL Delivery Vehicles (SDVs) used by the US Navy SEALs.
The US Navy’s four Ohio Class SSGNs are by far the most heavily armed conventional strike platforms in the world. They can carry a total of 154 Tomahawk Land Attack Cruise Missiles (TLAM Block-IV) in their missile silos. When added to the torpedo room, this gives a total of 176 full-size weapons. Although the maximum load is reduced when the DDS is fitted.
Iran has been exercising its first indigenous patrol submarine, the Fateh, in the area. This conducted a torpedo test, and was itself spotted by some analysts using OSINT (Open Source intelligence).
It is possible that the American submarine gained intelligence of the exercises or other events.
The Persian Gulf is very shallow in many places and generally seen as less suitable for large submarines. That does not mean that the USS Georgia cannot operate effectively in that environment however, but it may make incidents like this more likely.