There are no breakable walls, that I recall, and the only explosives in DD are for something else. The cracks are likely just for atmosphere / decoration.
Here is a map of the catacombs
level 4. If you can see rooms other than this, it is just a glitch with the map. There are many levels on each map as seen by the game, but normally areas you are not in get blacked out. Occasionally you can see into other areas. This glitch happened to me once;
Map (30KB) - showing several dungeons and sewers south of Rivertown.