The dungeons in Forager all have secret rooms in them. Most of these secret rooms contain one or more chests that tend to be quite valuable to open, so you should definitely learn how to recognize where to find the secret rooms, and how to open them. Most often these chests have a variety of gems and other valuable materials in them. There's even a challenge in the game that requires you to find all the secret rooms, and this challenge is not actually that difficult once you learn how to look for them.
So, here's how you find secret rooms in dungeons in Forager. Just look for cracks in the wall! All the secret rooms are hidden behind walls that are visually cracked. There is not a single secret room that does not have this, so just keep your eyes open for the cracks in the wall, and you will likely find them all.
To open the secret room, use an explosive on it. The easiest method is to use a Bomb, which can be crafted for 2x Jelly, 4x Iron Ingot, and 2x Coal at the Cauldron. Just make sure to craft 4-5 of these before you head into a dungeon, and you will be able to open all the secret rooms in it.
There are some other options for opening the secret rooms available a bit later in the game, such as the Fire Rod or the Ice Rod. You get these after doing the respective Dungeons, so there's no need to bother with Bombs after you have got one of these. In the late-game, you can also use your Bow with arrows that explode, or other means of explosions.