Official Blizzard Quote:
Diablo: Bubbling tar effect in early implementation state. Watch where you step.
Thus far, we've been shown deserts ( The Borderlands), dreary forests and hills ( New Tristram), an urban type area ( Caldeum), and various dungeons ( Tristram Cathedral). All of which have had a significant amount of concept art tied with them, along with screenshots for a select few.
But unless the streets of Caldeum are without any type of waste-management, where do tar pits fit into the mix?
The first environment that jumps to mind is of course, Hell. Now, we've mainly gotten bubbling lava for the other versions of it in both Diablo I & II, but I can easily imagine a tar pit, complete with unfortunate human souls trapped forever within its grasp, making a cameo in the Diablo III version of Hell.
Next up, is the Borderlands. This environment screams desolation through a megaphone. No doubt a pit of tar would go well with the tone set by the endless deserts, ruins, and dying vegetation. Also to take into consideration, is how these two elements have gone together in previous Blizzard games. Take Starcraft: Brood War, for example. The levels that took place on Korhal had their water replaced with tar, as well as the grass replaced with sand, ruins, and dying vegetation. It's a bit of a stretch, but connections can be made between the two.
Of course, there are many other possibilities aside from the two mentioned, but I'll leave the rest of the speculation up to you.