From creation.com:

“Carbon-14 is an unstable form of carbon that decays into nitrogen 14 at a measured rate-and this forms the basis of carbon-14 dating. In 2003 a group of researchers performed an unusual test on 10 coal samples obtained from Pennsylvania State University. The researchers wanted to see if carbon-14 could be detected in the coal samples. This test might be considered ‘unusual’ because carbon-14 decays relatively fast, and should not be detectable after a maximum of 90,000 years. Yet the coal samples tested came from strata allegedly ranging in age from 37 million to 318 million years. The laboratory results were clear: all of the 10 coal samples contained carbon-14; and similar amounts. This seriously undermines the evolutionary dates for the rock strata containing the coal, because the presence of carbon-14 affirms that the coal samples cannot be millions of years old. The results fit nicely with the coal forming from vegetation that was buried in Noah’s Flood.”