system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did mountaineers prove they reached a summit before the days of cameras and selfies?
gpt
Alpine and mountaineering clubs in Europe placed small vaults with rubber stamps and pads on the summits of mountains.  When climbers summited a mountain they stamped their climbing “logs” with that mountains particular stamp to demonstrate/prove their success.  Way cool that.