«Vitals» by Greg Bear

Scientist Hal Cousins is close to discovering the key to immortality but someone has already found it and will kill him to keep it secret. Vitals is a tense technothriller in the best Michael Crichton tradition.
A mile and a half below the surface of the Pacific Ocean, scientist Hal Cousins, frightened of the dark and no friend of God, is looking for the fountain of youth. The Nobel Prize doesn't interest him. Hal is in longevity research for the long haul, the really long haul. 'Angels' (rich businessmen keen to live a thousand years) fund him. Hal finds what he is searching for: xenos, the single-celled tramps of the sea floor, each one as big as a clenched fist. But then the pilot of his sub goes berserk. Hal barely survives; the xenos don't. The pilot kills himself. Five other scientists in related fields die violently in the space of a week. Hal discovers a trail of death stretching back over decades, from Stalin's Russia to present-day Manhattan. Another epidemic of murder by superbly trained killers has been triggered by what Hal nearly discovered…
From the bottom of Russia’s Lake Baikal to a billionaire’s bionic house built into the cliffs of the Californian seashore, from the darkest days of the reign of Joseph Stalin in Russia to the capitalist free-for-all of modern America, the edge of immortality is the most dangerous place to be.