The pilots may have only told half the truth. Mining / exploration firms use similarly shaped geophysical probes hung from the bottom of helicopters to detect magnetic anomolays associated with ore bodies. Saw one work around an old gold mine in Montana. Such exploration is usually 'hush-hush' since it is often on public lands and they don't want to tip off competing firms. So perhaps the 'mine' may not have been a mine in the nautical sense.
I saw a pic of one of those hanging off of a helicopter once.
So what do you think the Navy would have been looking for?