It occurs to me that just that one idea, miles of tunnels connecting separate dungeons, is a rich topic to explore.
For starters, suppose you've made and played a standalone dungeon. Now you've read Zak's post and said, "Damn, I want to do some of that!" You want to connect your dungeon to other dungeons, but you don't have any immediate ideas. You want a table for random creative input.
OK, how about this one?
|Level/Roll||Connection Type (Length)|
|2||Simple Tunnel (Leagues x 100)|
|3||Tunnel w/ Waypoint (Leagues x 100)|
|4-5||Tunnel w/ Level/2 Stages (Leagues x 100)|
|6-8||Tunnel w/ Level/3 Stages (Leagues x 10)|
|9-10||Tunnel w/ Level/3 Stages (5d6 Leagues)|
|11||Tunnel w/ Waypoint (2d6 Leagues)|
|12||Simple Tunnel (1d6 yards x 1000)|
- Count the number of levels in your dungeon.
- Roll 2d6. If the result is less than the number of levels, there is a connection to another dungeon on the indicated level.
- Look up the result on the table to get the type of connecting tunnel and length.
- Roll the dice indicated to get the exact length. For results below 9, roll 1d6 per level to get the length in the indicated units.
If a tunnel has stages (not a straight, uniform tunnel,) divide the level by 2 or 3 and round up to get the number of stages (2 to 4.) The first and fourth stages will probably be simple tunnels, but the second or third stages will each be a different environment: underground lake, canyon, flooded waterway, lavaflow, etc., any change of tunnel features that would require a change of equipment or transportation. When rolling the tunnel length, read the dice in groups of 2 or 3 dice, based the divisor. The first group of 2 or 3 dice will be the length of the first stage, the second group will be the length of the second stage, and so on.
If there is no ready-made connection point for your tunnel on the indicated level, then the tunnel is behind a secret door or a false wall. If you want your dungeon to be connected no matter what, then when you roll 2d6 in Step 2 above, if the result is not less than the number of levels, the connection is downwards to another dungeon via a chute or shaft; again, the chute/shaft can be concealed behind a secret door or false wall/floor, and the length will be a number of levels equal to the result of the roll.
For ideas about distance between levels and how far down you could go, see my old post about how deep a megadungeon could be.