Mayflower cleanup
National Wildlife FederationCleanup crews at a marsh covered with oil from the Mayflower spill in April.

The pipeline spill that flooded Mayflower, Ark., with up to 290,000 gallons of tar-sands oil in March was an accident that had been waiting to happen — for more than 60 years.

The pipeline that ruptured beneath the town was defective, with manufacturing flaws going undetected since it was laid in the 1940s, according to independent laboratory tests. ExxonMobil released a short summary of test results Wednesday.