A good way to rapidly acquire obsidian with little effort is to find or create a large pool of water on top of a similar-sized pool of lava, mine through the obsidian layer to create a tunnel straight down into the deep end of the lava, then mine a channel through the wall of the tunnel. Water will fill the channel and come into contact with the lava from below, causing more obsidian to be generated at the block that was just mined. This will keep generating obsidian until either the water drains completely or the lava drains to the level of the channel.
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