I used to be involved in repairs for undersea fiber...<p>Undersea fiber breaks all the time. When it breaks, you can send signals from the ends to find out where the break is. You then dispatch a ship to repair it.<p>However, if the cable breaks in two places at once, you cannot then detect a <i>third</i> break between the two you know about. That means someone who wants to tap your cable needs to break it in three places so that you aren't aware where they put the tap.<p>Surprisingly, it was <i>substantially</i> more frequent than random chance that a cable broke in two places in quick succession. To me that's good evidence that these cables were being tapped.<p>We also encrypted the data on the glass with dedicated per-link encryption units. I don't know if the adversary was hoping to break the encryption or had some way to extract the keys, or was just tapping the cable in case the data was unencrypted. I guess I'll never know.