Things we don't really want to think about

But with both nations keeping at least one nuclear-armed submarine constantly at sea for the past 40 years, he said it was no surprise that they had eventually ended up in the same area of ocean.

"Even in an ocean the size of the North Atlantic the submarines are eventually going to be in the same patch of water at the same time," he said.

[From British, French nuclear submarines collide - CNN.com]