Amazing Adventures#6(1961)

Our protagonist, an early SETI pioneer, has a great-grandfather who despairs from failing at creating a growth formula and so tosses it into the ocean. One hundred years later, our story begins where the vial is consumed by a small lizard-like creature. The missing ingredient for the growth formula turns out to be simply water and so Sserpo, as the lizard-like creature is named, begins to grow. The first humans to encounter him pull him into their boat with a net only to find that he is even bigger by the time they reach their island. Sserpo continues to grow until he makes the island sink and heads for Australia, nearly 1000 feet tall. The Australians use rockets to get Sserpo to turn back into the sea and he heads for Japan at nearly 2000 feet tall. The Japanese are worried that he will sink their island and so use an atomic bomb to frighten Sserpo off. The military are worried about his continued growth and so they attempt to slay him with a hydrogen bomb, but Sserpo just shrugs it off. All scientists are panicked, fearing Sserpo will grow so large he will upset the orbit of Earth, but our hero refuses to give in to fear and doggedly continues his attempts using radio equipment to contact the largest planet, Jupiter. He succeeds, and the Jupiterians send a skyhook across space to snag the now-visible-from-space Sserpo off the planet, saving Earth.