500
What event does this quote refer to:
"A lot of them died there. The earth, now warm, shifted; the first forepole slipped; loose rock fell from the face of the tunnel and caused a shield to give way. They found themselves in a chamber of water, deprived of the sun that had brought them there. With the first crack and whoosh of water, the clamber to get out was so fierce that others who were trying to help were pulled to their deaths."
What is National Suicide Day 1941