Here is inspiration for your comic from my Dragon Magazine, January 1994 article on pages 99 to 101 and my letter on page 55.
https://annarchive.com/files/Drmg201.pdf
Example:
Let’s take a look at an unusual real world landscape and transform it into a fantasy campaign setting. Virginia’s Natural Tunnel is a limestone cave formed by Stock Creek. The cave is 850’ long, averages 100’ high, and is between l00’-170’wide. Outside the cave, the creek has also cut cliff walls around itself that are between 350’-700’ high. Stock Creek does not fill the entire cave floor. A railroad was built in the tunnel in 1890 and is still in use. In 1971, the cave and the area around it became a state park.
The PCs are rafting a river in an area that’s recently been invaded by evil forces when a sudden storm strikes. Just as the party decides to land and wait out the storm, they round a bend in the river and see the water flowing into a large tunnel-like cavern. There also seems to be a rocky path or road leading into the cavern. Will the PCs explore the cave? Will merchants, bandits, or dragons also try to avoid the storm by taking shelter in the cave? Could this area be sacred to the local human or humanoid population? Could there be a sawmill, dwarf village, tomb complex, or winery within the cavern? What is at the other end of the cavern?
For over 400 years, the slow-moving river was the cheapest, fastest way to move goods and people between two human nations in the area. Along the river, canals were built to go around rapids. These canals also help boats and barges overcome the river’s slow current. Mules and horses are used to pull boats and barges in the canals and through their locks, (Ambushing the PCs at a canal lock would make a quick mini-adventure.) For a safe rest area, a village was built inside a huge river-cut tunnel.
