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Lonelycache.com - a website to look up the least-often-found caches in any US county!
(www.lonelycache.com)
Geocaching is a worldwide scavenger hunt that uses a GPS or phone app with GPS. There are over 3 million active caches around the world. Some are as small as a dime, others are big enough to walk into, all of them provide a unique adventure. We invite you to share your stories, photos, and experiences!
Is there a reason it's US only? API limitation or just scope chosen by the developer.
Think it is a choice by the developer.
But Project-GC has similar tools here https://project-gc.com/Statistics/LonelyCaches and here https://project-gc.com/Statistics/DaysSinceLastFound. Maybe not as nicely presented as the site in the OP, but can be used for all the countries.