[ Probably just as terrifying is Spock's manner in talking about it so blandly ... ]

By the use of a material simply known as "red matter". As a means of revenge for a perceived slight, a Romulan known as Nero used his mining vessel to drill a hole into the planet and injected the red matter, causing it to ignite at the core. The resulting singularity consumed the planet and its six billion inhabitants. There were only approximately ten thousand survivors.