The third annual Mercer Island Letterboxing Adventure in Pioneer Park is coming to a close on Oct. 19. Letterboxing is an intriguing mix of treasure hunting, art, navigation and exploration. Participants learn about the environment of all three sectors of Pioneer Park, where the letterboxes were hidden.

A committee of resident volunteers and staff from the city’s Parks and Recreation Department and the Mercer Island Boys and Girls Club planned and implemented the program.

Over 400 clue booklets were distributed to children in kindergarten and elementary schools. Many have turned in their clue booklets containing the stamps they obtained at each of the letterboxes that they found. The deadline for the submission of booklets is Oct. 18. They can be dropped off at the reception desk at the Mercer Island Community and Event Center or the Administration Building at Luther Burbank Park. Booklets are not required to have stamps from all sites to be turned in for recognition.

Letterboxing is sponsored by the Mercer Island Open Space Conservancy Trust (OSCT).

All participants are invited to a celebration at the start of the Open Space Conservancy Trust’s next meeting on Nov. 8 at the City Council Chambers. For further information, contact Elliot Newman, OSCT chair, at eznewman@yahoo.com.