I'm not a hugely experience minecraft player, I've never built anything really cool in singleplayer mode and I've not even found diamonds before, but I decided I was going to finally do something cool and build an awesome highway in the netherworld and then build another portal in the nether to get me to somewhere crazy and distant in the main world.
Eventually after exploring numerous horrifying caves I found one that went deep enough to find diamonds, and after a whole series of terrible events where I lost all my torches or fell in the lava through a 1 square wide hole in the floor I eventually got my diamond pick and frantically mined a bunch of obsidian in the half-dark monster filled cave. I decided to cut my losses and just build the first portal in the middle of a frozen lake using lava and water and a mould made of dirt.
After being blown up by horrifying giant faces I constructed myself a nice little fort in the nether, then I set out to plot the course of the highway using torches. I climbed down and down and eventually discovered an awesome arm of netherrack stretching out over a lake of lava with a bunch of streams dropping down around it. I built the portal at the end of it with the intention of building the highway later. But before I could go through it a fireball blew up the ground I was standing on and sent me plummeting into the lava.
I respawn and go back into the nether with some basic supplies because I really want to see where the new portal goes. As I warp through a huge pale face looms up behind one of my fort's narrow windows, and shoots a fireball at me. I move but it hits my portal and breaks it. I freak out briefly because I forgot to bring a flint, but then I remember the other portal. I can just go through there and find the materials for a new flint, then come back and fix my old portal.
I jump into my new portal eagerly, but reappear in total darkness. The portal is in the middle of a deep and dark cave, deep enough for veins of gold ore to line the walls. I start setting out torches, hoping to find the way out soon, but I run out long before I see anything that looks like the way up and out. I'm lost, and the zombies just keep coming out of the shadows, and my sword doesn't have much durability left.