Underground got a trailer at E3 but the same week the developers let a bunch of folks play the mode and upload content to Youtube.
Short version:
- they are infinitely replayable, randomly designed missions and you can choose if you want them to last 1, 2 or 3 stages long (with each presumably offering better rewards);
- you can select modifiers that make the mode easier or harder (example: if you reload before your clip is done, you lose the unused bullets), with modifiers changing the rewards as well;
- for Xbox users, this launches at the same time as update 1.3 which sees a lot of changes including new gear sets, a new heroic mode (more difficult than challenging) for incursions and missions, massive weapon balancing, new higher powered weapons (229 for weapons and 268 for gear score), new weapons, new weapon talents, etc.