Minecraft wiki entry for Nether_portal, associated text: When a player in the Overworld or the Nether stands in a nether portal block for 4 seconds, the player is taken to the other dimension. The player can step out of a portal before it completes its animation to abort the teleport. However, in Creative, the wait time is one game tick (1⁄20 second) for the player to transfer between dimensions. If there is already an active portal within range (about 128 blocks) in the other dimension, the player appears in that portal. Otherwise, a portal is created at or near the corresponding coordinates. If a portal is deactivated, and the matching portal in the other dimension is used before it is re-activated, a new portal may be created (unless there is another active portal within range). The usual cause for this is when the player's Nether-side portal is deactivated by a ghast, and then the player dies in the Nether, spawns, and then re-enters the Nether through the Overworld-side portal. However, multiple portals can be exploited to farm obsidian.