The countries that do not have snakes are Ireland, Greenland, New Zealand and Iceland. The continent of Antarctica also does not have snakes.
Scientists believe that during the most recent Ice Age, which took place roughly 10,000 years ago, Ireland's climate became too cold for snakes and many other animals to survive, so they either left the island or they died there. When the Ice Age finally ended and the climate returned to normal, Ireland was surrounded by water, which cut off the snakes' access. Snakes are notoriously slow to adapt themselves to new environments and climates, which may explain why any snakes that have been introduced to Ireland since the Ice Age ended have not been enough to recolonize the country.Although New Zealand has no land snakes, it is visited by two species of sea snakes. Sightings are rare; the snakes are poisonous but extremely small and are not regarded as a threat to humans.