Underworlds has board game-styled boards, which is nice for quick setup, but far flatter than I’m used to. Fortunately, starting with Underworlds, GW has released plastic terrain kits with pieces for each of boards’ special hexes.
For blocked hexes, it’s very useful to have the terrain as truly blocked. For some of the hazardous hexes (the tentacle pit of doom), the piece is flat enough for a model to stand on. For others, you’ll need to move the marker off the hex. That makes them somewhat less useful and immersion-breaking. Ahh, well, it’s better than nothing.