HELL'S GATE, Kenya — Two-and-a-half hours northwest of Nairobi by car, a small group of bitcoin miners set up shop at the site of an extinct volcano near Hell's Gate National Park.The mine, tucked away on the edge of Lake Naivasha, is operated by a startup called Gridless and consists of a single 500-kilowatt mobile container that, from the outside, looks like a small residential trailer.Backed by Jack Dorsey's Block, Gridless electrifies its machines with a mix of solar power and the stranded, ...