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. 2017 Jun 22;8:15872. doi: 10.1038/ncomms15872

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Copyright © 2017, The Author(s)

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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(a) A high-resolution side-scan sonar image showing a type of polygonal structure, which represents a regularly shaped depression surrounded by a raised shaft; (b) a high-resolution side-scan sonar image showing a type of polygonal structure characterized by a regular pattern consisting of raised oval-like shapes.