Estimated old ages for the Earth are frequently based on “clocks” that today are ticking at extremely slow rates. For example, coral growth rates were thought to have always been very slow, implying that some coral reefs must be hundreds of thousands of years old. More accurate measurements of these rates under favorable growth conditions now show that no known coral formation need be older than 3,400 years.a A similar comment can be made for growth rates of stalactites and stalagmites in caves.b [See Figure 153 on page 262.]