It seems that every month brings news of yet another overlay network. STT, OTV, VXLAN, NVGRE and may be many more I've not heard of. People often use "ships in the night" words in the same sentence referring to the fact that these overlay network know nothing about underlying physical network infrastructure. However, nowadays even the smallest sea or ocean going vessel not only knows about any ship nearby, but acutely aware of a few satellites thanks to radar and GPS systems.
So, to keep up with pace of time and advances in technologies (after all, we are in tech business) I propose new analogy for overlay and physical networks: polar bear and penguin. Those 2 definitely do not meet in real life. I am not considering corner case like zoo.