Official websites use .gov A .gov website belongs to an official government organization in the United States. Secure .gov websites use HTTPS A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites. National Park Service Search Exiting nps.gov Rate the Lesson Plan Student Activities Cardboard Caving Grade Level: Upper Elementary: Third Grade through Fifth Grade Subject: Science State Standards: ESS2.C ESS3.C Materials: For the cave: various cardboard boxes (large and small), utility knives or a serrated blade (i.e.