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Top Holes - Chukchi Sea exploration

Wells, like houses, begin with a foundation. In the Arctic, the foundational well -- also called a top hole - starts with a 20-by-40 foot area excavated into the seafloor called the mud-line cellar.

This hole is an insurance measure to protect the blowout preventer (BOP) from the risk of damage by any large, unseasonal ice floes. During the majority of the season, the wells Shell plans to drill are in ice-free waters. However, right at the end of the season, ice will begin to re-form near the prospects and the mud-line cellar provides an extra level of protection against any ice floes that could reach to the sea floor.