ON MARCH 4, an embryologist at Pacific Fertility Center was doing a routine walk-through of the clinic’s collection of waist-high steel tanks, each one filled with thousands of liquid nitrogen-bathed vials of frozen sperm, eggs, and embryos. The San Francisco-based clinic offers cryogenic cold storage and in vitro fertilization services for patients throughout the Bay Area, many of whom work for tech companies with hefty fertility benefits packages—Apple, Google, Facebook, Pinterest, LinkedIn. PFC charges its patients $600 a year for storage alone, which covers the personnel required to maintain the tanks, according to its website.Every day someone has to do a physical inspection of the equipment, and staff are on-call 24/7. But that Sunday, the embryologist discovered that in one tank, Tank No. 4., the liquid nitrogen levels had slipped to dangerously low levels.