This is not the first room I have made, just the first to be photographed in depth. This one was done as an entry in a Terregenesis competition for "dangerous ground." The special rule for this room is that any time a Hero fails to escape a pit trap card they are instantly killed.
The bridge came from a curved section of packing foam that protected a ceiling fan. It had the right amount of curve to fit within the 6 inches on this room, and wasn't too steep -- I did have to cut some of the angle out on the second "square" of the bridge to make it easier to stand miniatures on that space.
The cracks were cut out of the thick layer of cardboard, and I used spackling to fill in the corrugations and make it look more like rock (I'm pretty pleased with the results). Looking at pictures of lava flows, I notice that rough areas were common as well as the smooth areas where bubbles had solidified. I used sand for the rough areas, but I didn't do as well with the smooth areas, just used scraps of card and dry-brushed them a lighter color.
Painting was fun. I tried to use orange to highlight some "hotter" areas in the lava cracks. I used gray and tan dry-brushing to highlight the sand and used brown to try to outline the different areas of the "rock."
The bridge, being foam, was painted with a latex gray, rather than black spray paint, and so it is a different shade than the five floor tiles on each edge. I used a black wash to emphasize the squares and cracks, and then dry-brushed all the squares (foam and card) with dolphin gray to try to make them blend together (partially successful).