There are two IM styles. One has a long staircase and is usually found along or near the coast. The other looks like a dvergr fortress and can be anywhere in the interior. As others have said, the Mistlands forces us to rely more on hearing than vision and we can usually hear a seeker mob that congregate around a IM. Worst case, use the generator[valheim-map.world] to load your seed and see if the landmass you are currently in has any IM at all. I suspect the developers did listen to some of the feedback since the number of black cores in the IM seem to have increased so even if you locate one you have a good chance of crafting the Mistlands crafting stations. The feedback was that with 1 or 2 cores in an IM, you nearly defeat the entire Mistlands before you can start crafting Mistlands gear. On the other hand, the fragments of the seal breaker seem to have gone down and that seems fair. You still have to explore up to 10 IM to be able to go engage The Queen but at least in the later IM you go with Mistlands gear. This was excellent work from the developers.