The Friends of Upton State Forest and the town’s Open Space Committee are sponsoring a free letterboxing hike Aug. 19 at 1 p.m. (rain date: Aug. 26, 1 p.m.). The hike will travel through Upton State Forest to five letterboxes placed in the 1930s at points between the Civilian Conservation Corps camp and Dean Pond. Hikers will learn about the CCC’s history at Upton State Forest along the way. The hike begins at the former CCC headquarters building near the intersection of Westborough and Southborough roads (the GPS address is 205 Westborough Road). The moderate hike includes uneven terrain and some hills. The long hike option follows trails to Dean Pond and visits all five letterboxes. A shorter hike that includes three letterboxes will also be offered. Each hike is expected to take between two and three hours to complete. Hikers should preregister by e-mailing uptonopenspaceevents@charter.net, or by calling 508-529-6610. Preregistered hikers will get weather updates and other notices by e-mail.