Deep in the Gulf of California, scientists have discovered a fantastical expanse of hydrothermal vents, full of crystallized gases, glimmering pools of piping-hot fluids and rainbow-hued life-forms. Punctuating it all are towering structures made of minerals from the vents, looming as tall as 75 feet (23 meters). A decade ago, scientists visiting this spot saw nothing unusual; this psychedelic seascape seems to have built up around an increase in hydrothermal venting — spots in the seafloor where mineral-laden and superhot water jets out — in the last 10 years. "Astonishing is not strong enough of a word," said Mandy Joye,...