It may seem a little presumptuous to call my ancient military complex a megadungeon. At the present, I think it only has about a hundred, maybe 150 rooms that the players know about- surely this is too small to qualify? The key to that statement are the words "at the present". There is plenty of room to expand, and I have already planted the seeds in the players' heads that will eventually lead to the exploration of three more large areas. And as play continues, I am sure I will plant more seeds and more areas will be discovered. As it is, the first time the players ran through the complex they spent several sessions searching for the gold key cards that would allow them to access some of the sealed off areas. When they finally did find them, they ended up getting chased away by some Warangutans and haven't returned yet. This means that after six straight four to six hour sessions, the players still have only scratched the surface of the complex. The door temptingly labeled "Alien Weapons Division" is still unopened! Likewise, the top secret area that they learned about in Julius Corple's papers is still virgin territory (an upcoming post will detail the tragic story of Julius Corple, brilliant roboticist and the unwitting cause of the downfall of the military complex at the hands of the Oozyxphg). The players turned tail and ran from one of the largest levels of the complex before they had even really began to explore it, because they realized it was the home to two groups of warring, possibly partially robotic killer apes (think Planet of the Apes meets Vietnam with a little bit of Robocop thrown in and you have an idea of what that level was like). There are several long tunnels leading from the lowest hanger level, one of which evidently leads up to the swamp because it was flooded and nearly drowned the players when they opened it; it took some pretty heroic and nearly self sacrificial actions from two players to save the rest of the party from a watery grave. However, the other tunnel leads to the labyrinthine science complexes underneath the old university inhabited by the Ooh Oh Monks, and offshoots lead to places as yet unknown to the players. Only a handful of the officers' and scientists' quarters have been explored, as the players mostly focused on those that they suspected might contain a gold key card. All in all, the campaign could easily go on for a long long time in this one network of underground chambers, and that makes it a megadungeon in my book.
One of the things that James over at Grognardia identifies as a key feature of a classic megadungeon is that it is dynamic and changes in response to the players. My little complex is an excellent example of this; play began with it flooded with poisonous gasses, so besides the odd alien critters that had come through the portal from the Oozyxphgian homeworld, it was mostly robots that the party had to deal with. However, these robots were not sitting idly by waiting to be discovered. The still active security cameras sweeping the corridors detected the party's movement despite some interesting efforts to circumvent them, and a team of robotic defenders was dispatched.
When the party eventually shut the gas off, the apes and other denizens who had been trapped in self-contained areas of the complex that the gas had not been able to reach soon emerged and this really changed the dynamic of the area. As the party was trying to make it to the armory that they had seen on a building schematic in a computer they hacked into, they realized that they had been beaten there by the War Chimps. They watched through a camera feed with a mixture of curiosity and horror as the War Chimps easily handled several Scorpions and took control of the coveted armory.
I have always been a big fan of moving monsters around the dungeon; it prevents complacency by the PCs and it makes the dungeon feel "alive". The most recent example of this from my campaign occurred during the last session. The party has been out in the eastern desert involved in some tricky diplomacy in an effort to prevent the Knights of Genetic Purity from gaining access to nuclear weapons controlled by three oft-times warring desert tribes. During the last session, the Swamp Orca (the party's hovercraft, purloined from the hangar level of the megadungeon and outfitted with Glarg-ore armor courtesy of the Badder master smiths) took some serious damage and was on the verge of a total breakdown. The skirt that contained the cushion of air had been badly ripped, so Logan the incredible teleporting shrinking man teleported back to the hangar level to scavenge sections from the skirts of the other ruined hovercrafts that they had left behind. When he arrived, he noticed that the level was being patrolled by the Warangutans. He also noticed that the massive blast door which sealed the hangar off from the tunnels leading to the Ooh Oh Monks and the Badders' tunnels was wide open, and a large pile of Badder corpses was stacked near the tunnel mouth. The party correctly deduced that the Badders (who the party had given the code to open the door) had attempted to scavenge more of the ruined vehicles in the hangar to offer as scrap metal to their lord Glargorion and had run into the Warangutans. As several of the party members were quite taken with the great worm god (Dataan has been spreading the gospel of Glargorion wherever he goes) and the entire party was quite grateful to the Badders for their assistance in crafting Glarg-ore weapons and armor for them, the last session ended with talk of returning to the military complex to deal with the Warangutans. Having things occur in the absence of the party is a useful tool, and one that greatly adds to immersion in a game world.
Stay tuned for more lessons from the megadungeon, including the tragic tale of Julius Corple!