Perforated Shells (IMAGE) University of Bristol Caption These are perforated shells from level II of Cueva de los Aviones after cleaning. Credit PNAS Usage Restrictions For single use only to illustrate this story. Not to be archived. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.