BOPs = Blow out Preventer. They come in various configurations, but they will have various chambers and mechanisms that “grab” the pipe or tubing.
You set them up in a stack that do different things -— rams, different sizes to grab different sizes of pipe, diverters, even ones that cut the pipe and shut it down.
When you are underbalanced (when the well is coming to see you due to lack of head pressure or a gas bubble or somesuch) the BOP should stop this or be able to divert it somewhere where it can be flared relatively safely (on land, over the pit or something).
Anyway, there are a host of different ways BOPS can be closed or a well diverted, just depends on what happened and your set up.
Here, they will probably send in underwater robots (again, depending on location of the BOP) to close it off.
Thank you very much! I’m happy to learn something every day.