I had never heard of Nimrod Springs although it's not top secret or hard to find.

You'll find it on the way to Missoula and is worth a stop if you're already in the area, but not necessarily reason enough to hop in the car right now.

There IS an underwater cave that you can get to, but unless you are super comfortable and knowledgeable about hot springs, you might want to skip that part. (This video DOES EXPLORE the cave, so at least you'll be able to see what it looks like.)

Since the hot spring pool itself is rather large, consider bringing a snorkel mask with you to explore the super clear area.

BE CAREFUL WHEN PARKING YOUR CAR! There is an approved "lot" about 1/2 mile off the road but it's MUCH safer. (Just west of the Bearmouth exit on I-90).

Find out more details about Nimrod Hot Springs here!