Ultima V introduced the vast
Underworld to
Britannia. Where in
Ultima IV the game world consisted of the
continent of Britannia, its nearby
islands and
dungeons, with Ultima V
Origin Systems basically added a second world underneath Britannia that is comparable in size. This world was accessible by
exits at the bottom of each of the dungeons, or through a couple of key locations topside.
The Underworld in Ultima V is particularly nasty; it's best described as a large outdoor dungeon. It's dark pretty much everywhere, requiring a steady supply of supplemental light. There are plenty of dangerous monsters down there, so you spend time getting beat up. A lot of the underworld is made up of this twisted maze of mountains that you need the grappling hook to traverse, during which you usually take a healthy amount of damage. There exists a plethora of one-way rivers and river-waterfall combinations making it near-impossible to remember how to get from one location to another, and removing the option of backtracking. There's nowhere to buy food or supplies and you're constantly getting beat up by either the landscape or the monsters; this is not a very fun place to be.
The underworld doesn't come with it's own cloth map, you have to figure it out yourself. The three shards you need to defeat the Shadowlords are located at various places in the underworld, as are several other objects you need to recover.
Ultima VI followed in this idea by introducing the Gargoyle world counterpart to Britannia, but you don't return to the Underworld.
I also believe that Ultima V was the first of the Ultimas to allow the user to fire weapons and spells in directions other than the usual up, down, left and right.