Complex sentence: Experts, scientists, and the United States military fail to discover the cause, though one scientist suspects radioactive contamination from a space probe returning from Venus, which was deliberately exploded in the Earth's atmosphere when the radiation was detected.
Split into simpler sentences: Experts, scientists, and the United States military fail to discover the cause, though one scientist suspects radioactive contamination from a space probe. It returned from Venus, and deliberately exploded in the Earth's atmosphere when the radiation was detected.