Tutorial:Nether portals

The creation of Nether portals can be used for a variety of different means. This page lists some of the implications of these portal mechanics.

Portals try to avoid spawning over lava, in midair, or inside rock, but they do so by spawning nearby. Thus, a new portal from the Overworld has a disproportionate chance of being next to an abyss, lava lake, or netherrack wall. There is also no way to check whether a lava source (created with the landscape) is destined to send lava flowing over the portal. Furthermore, a portal can spawn on a one-block thick ledge or floor, or on a Soul Sand outcrop.

Portals built in the Overworld should be secured in order to prevent wandering monsters from entering the portal. These creatures can accumulate over time in the Nether; not only is there no sunlight to burn undead, but without a player present, time barely passes for them (15 seconds for each new entry) so they may not have time to despawn. Mobs such as creepers are especially bad to handle in the Nether due to the soft nature of netherrack and the abundance of lava to fall into.