"Earthwelling" is the very simple process of turning lava into obsidian deep underground with the help of water and a pickaxe strong enough to cut through obsidian (of course).
Now before you start thinking, "What will I ever need obsidian for?", there are many reasons. And when I mean many, I mean about 3. You can make building material, you can make obsidian skulls, and you can make a legendary sword that does 999 damage. No, I'm just kidding. Many people make obsidian skulls to sell, which with the help of 20 obsidian, you can sell for a profit of 54 silver each. Doesn't sound like much, but once you really get into this trick, it will start to sound like it. Now of course there are other uses, like protection against fire damage ( Not including lava ), But this is the most used. When you are "Earthwelling", you want to go deep enough underground to the point where there are many pools of lava and many pools of water. Once you find your spot, make a tunnel from a large pool of water, to a large pool of lava (You want your pool of water to be bigger than your pool of lava. The bigger it is, the more lava pools you can tunnel to, which means more obsidian.) Let the water flow to your lava pool, note that drowning is inevitable(Certain to happen) if not cautioned. The water will turn the top layer of lava into obsidian, mine it quickly and breathe for air. Once that pool has been completely harvested, if your water pool is big enough, move to the next. Keep doing this trick and you will start making gold in no time. You may like this trick if you don't like cheating. Although it is a way to harvest money the creator may have not intended for, and is an easy way to make money, it's not a glitch (Which I myself like about it). It's more fun than doing glitches like the Gold/Shadow chest duplication glitch. Here is a list of sources and videos.
Credit to www.Terraria.wiki.com - http://terraria.wikia.com/wiki/Obsidian_Skull
Credit to "theflokin" on www.Youtube.com - http://www.youtube.com/watch?v=SIsXRPUEMRc
Hope you enjoyed this, have a good day! Kick some butt in Terraria!