This Hacker News thread discusses a recent XKCD comic titled "Holes," which explores remarkably deep human-made structures and natural geological features. Participants shared fascinating examples of Earth’s depths, including the Cave of the Crystals in Mexico, Lake Baikal’s extreme sediment depth, and the massive salt mines beneath Detroit and Yerevan. A significant portion of the discussion focuses on the Kola Superdeep Borehole and the Mponeng Gold Mine, comparing the logistical challenges of deep-earth operations with the dangers of drilling accidents, such as the catastrophic 1980 Lake Peigneur incident. Users also debated the technical nuances of borehole depth versus horizontal reach, often clarifying misinformation regarding records held by oil companies like BP. The conversation touched upon the human tendency to seek patterns in geological data—noting that the coincidental similarity in the depths of certain boreholes often leads to wild speculation. Ultimately, the thread serves as a crowdsourced exploration of the planet's hidden depths, with users exchanging links, geological trivia, and personal recommendations for games and documentaries related to subterranean exploration.