This event is in the "Everyone" group. BPL Letterbox Challenge 7:00am - 8:00pm Everyone Offsite Event This event is in the "Everyone" group. Aug 28 2020 Fri BPL Letterbox Challenge 7:00am - 8:00pm Bloomington Public Library Please note you are looking at an event that has already happened. Library Branch: Bloomington Public Library Room: Offsite Event Age Group: Everyone : Letterbox Challenge You may have heard of geocaching, but have you ever heard of letterboxing? Letterboxing is a treasure hunt in which participants locate boxes which have been hidden throughout the community.