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It is because Corregidor is dominated by Alas-Asin at the end of the Bataan. Anyone controlling Alas-Asin can easily shell all of Corregidor. Originally the US fortress on Corregidor (Fort Mills) had only surface fortifications, but the Army realized that the fort was completely vulnerable to a shore bombardment. Therefore, to make the fort a "self-sufficient" bastion, the tunnels were dug, potentially allowing the US forces on the island to resist a siege, at least temporarily. These preparations later proved to be very useful, because the island was able to hold out against the Japanese for four months during World War II.