I found Manresa Grotto by accident. I was driving from Newport to Metaline Falls, but Highway 20 was closed at Lost Creek, so I had to cross the Pend Oreille River at Usk and drive up the east side. That was a happy detour -- I saw a herd of buffalo and chanced upon Manresa Grotto.
When I got home I Googled it and found this Pend Oreille Heritage website. http://pendoreilleheritage.com/?page_id=15&location_id=360832
The cave is a natural feature that was used as a chapel beginning in the 1840s, when a Jesuit missionary arrived in the area. He called it Manresa Grotto in reference to a famous cave in Manresa, Spain, where St. Ignatius of Loyola is said to have prayed. a stone altar remains in the grotto. Sadly, the grotto is marred by graffiti. Still, it's an interesting site geologically and historically, and well worth a visit.
The trail up to the grotto is well maintained. The steps and the floor of the grotto are very uneven, so take care walking inside, especially when your eyes are not accustomed to the dark.
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