Minecraft Nether portal in cavern?!?

this is freacking me out! i have never built a nether portal EVER! im mining and find a HUGE cavern that seemes to never end. (not 404 seed) then i hear the noise that the portal makes so im like wtf? i go further in to the cave and feaking see a nether portal! a random portal in a dark cave with like skellies around it. it is single player meaning no online people in it and i have NEVER build one of those portals. has this happened to anyone else? do the maps generate portals? IM FREAKING OUT!