It takes ten obsidian to make a Nether portal. Once you have enough for two portals, however, you can use a trick to get infinite obsidian without needing to find lava.
 If you don't have a portal already, place obsidian blocks in a vertical frame 5 tall x 4 wide. Activate it by using a flint and steel on the lowest obsidian block. This trick may not work if there is another portal close by.The corners of the portal do not have to be obsidian.
 The Nether is a dangerous place, so prepare yourself if you haven't been before. You'll need the remaining ten obsidian blocks, but you might want to leave them safely behind and explore a safe route first. You'll need to travel a certain minimum distance in a straight, horizontal line (these numbers include a 3-block margin of safety just in case):PC, Pocket Edition, and Console Edition "large" worlds: travel 19 blocks.
Console Edition "medium" worlds: travel 25 blocks.
Console Edition "classic" worlds (including all PS3 and Xbox 360 worlds): travel 45 blocks.
If you have multiple Overworld portals, walk away from their coordinates. This trick will not work if you are too close to an existing portal.
 Build this in the Nether and activate it the same way you did the first one. When you walk through it, you should appear in a brand-new portal in the Overworld.


If you appear next to a portal you already built, you did not walk far enough in the Nether. Go back to the Nether and break your portal with a diamond pickaxe, then rebuild it somewhere else.
 The portal that just appeared has fourteen obsidian blocks free for the taking. Mine these with a diamond pickaxe.
 Each time you walk through the Nether portal you just built, a new portal will appear in the Overworld. Mine this for free obsidian. Speed this up if you want a large amount of obsidian:


Use a bed to set your spawn near the permanent Overworld portal.
Place a chest near the temporary Overworld portal. Stash the obsidian and diamond pickaxe in the chest after mining the portal.
Kill yourself to return to spawn.
Walk through the Nether again and exit the same portal to create a new one. Build a tunnel between the Nether portals to increase safety.
Gather twenty obsidian by other means.
Create a Nether portal.
Travel through the Nether.
Build a second portal.
Mine the obsidian in the Overworld portal.
Exit the same Nether portal to spawn a new one.
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