Complex sentence: The London deep-level shelters are eight deep level air-raid shelters that were built under London Underground stations during World War II. Each consists of a pair of parallel tunnels 16 feet 16 inches in diameter and 12,000 feet long.
Split into simpler sentences: The London deep-level shelters are eight deep level air-raid shelters that were built under London Underground stations during World War 2. They consist of a pair of parallel tunnels 16 feet 16 inches in diameter and 12,000 feet long.