About 65 feet below ground level in Paris is a labyrinth of centuries-old tunnels known as the Catacombs, which hold the skeletal remains of around six million Parisians. Legend has it that plague victims comprised some of the bones lining the hallways. This week, it turned into a techno cave with neon lights for top ethereum programmers to party till 4 A.M. The mantra was “leave no trace” so everything was gone by sunrise. “It’s very illegal but also very crypto,” explained one of the roughl...