On land we have them secured - solid rock from the bottom, national air defense from the air, and big security teams + police and even military forces ready to react at minimal notice.

Hijacking a plane that is traveling between second and third world countries is plausible - MH370 showed that there are deficiencies in aircraft tracking. And while submarine attack is not trivial it is not totally impossible.

And if we have a couple of years down the road a situation like the current - with free falling oil prices, there are state actors for which some disruption will be welcome.