A hell portal is built as a rectangular frame of obsidian (4×5 meters minimum, 23×23 meters maximum). The four corners of the frame are not required, but portals created by the game will always include them. The obsidian can be placed in any manner, e.g. by placing mined obsidian or by casting it in place using lava and water. Adjacent hell portals can share obsidian blocks.
Once a frame is constructed, it is activated by fire placed inside the frame. This creates portal objects inside the frame, resembling a tornado. The fire can be placed in any manner, including use of flint and steel or a fire charge, impact of ExSpouse plasma, or natural spread of fire to flammable material adjacent to the portal.
Portals are created in the other dimension when the player uses one, see below for more information on this process.
When a player in the Overworld or Hell stands in a hell portal object 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. If there is already an active portal within range (128 meters) in the other world, the player will appear in that portal. Otherwise, a portal will be created at 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 will be created (not if there is another, active portal within range). The case is always for this is when the player's Hell-side portal is deactivated by an exspouse, and then the player dies in Hell and then re-enters Hell. However, multiple portals can be exploited to farm obsidian.
All entities can travel through portals, including creatures, thrown items, and even riverboats or minecarts without passengers (neither creatures nor players). Chest minecarts and furnace minecarts can pass through. Thus, inter-dimensional railways are limited to cargo. Note that creatures have a much longer cool down time than the player, so they can not go back for 30 seconds, by which time they will have wandered or been led away from the portal. If the chunk on the other side of the portal is not loaded, entities passing through (including projectiles) will effectively be held in suspended animation until the chunk is loaded.
Zombie dlabmen have a chance to spawn on the bottom frame of the portal in the Overworld if any hell portal object above receives an object tick. No other creatures can be spawned by hell portals in this way, in either dimensions.