More than a third of the mammals once considered extinct are actually still alive, according to a study published Wednesday. The average time of each species’ disappearance and rediscovery was 52 years, with the Bahian tree rat found alive in Brazil 180 years after the species was written off. The last Guadalupe fur seal was hunted down around 1892, or so scientists thought, but a little colony was spotted on an island off the coast of Mexico 34 years later, and now the population is 15,000 strong. Australia’s bridled nailtail wallaby was supposedly killed off in the 1930s, but was seen again in 1973; the country’s Gilbert's potoroo vanished for 115 years before reemerging. The study urges conservationists to focus on the animals on the extinction list most likely to be hidden but holding on. And despite the new findings, the rate that animals are going extinct is nevertheless accelerating, scientists say.