Stripe is one of those technology companies that controls the internet’s plumbing. It makes payments-processing software that hustles money from your debit or credit card to someone else’s bank account. If you’ve ever purchased groceries on Instacart or supported a project on Kickstarter, you’ve used Stripe, even if you didn’t know it. Owning this particular corner of internet infrastructure is highly lucrative. Stripe is worth $36 billion by one metric, making it among the most valuable U.S. start-ups that have yet to go public. Only a handful of firms, such as SpaceX and Cargill, are more valuable. Lately Stripe has been helping to build a different kind of plumbing—physical pipes running from the open air to deep underground.