Complex sentence: In October 1917 the United States Navy tendered an order for the 100,000 mines necessary to lay a minefield stretching 230 miles and dangerous to a depth of 200 feet.
Split into simpler sentences: In October 1917 the United States Navy tendered an order for the 100,000 mines. This number was necessary to lay a minefield stretching 230 miles and dangerous to a depth of 200 feet.