In 2022, Mark Monroe's podcast about hydrogen was one of our most popular podcasts ever. At Microsoft, Mark had just made a prototype hydrogen fuel cell UPS system that could potentially replace diesel generators to provide low-carbon backup power at data centers. One year on, he's back for some more detail. A hydrogen economy will need a distribution system - but will that look like a power network, a gas grid, or a system of trucks?  Data centers won't be the first big users of hydrogen: Mark tells us where it will take off.  This year, we've heard stories of natural hydrogen mined from underground. Mark assesses that prospect, along with the other sources of hydrogen, and suggests that the new energy source could rewrite the world's map of energy providers. Listen to our talk with Mark to find out where and when you will start using hydrogen.