Here is a concise summary of the provided text:
**Title:** Unexplored Ocean Mysteries: 7 Terrifying Creatures and Sounds
**Summary:**
* The ocean remains largely unexplored, with <5% of the seas discovered, hiding thousands of unknown species.
* This article/video counts down the top 7 most terrifying and mysterious creatures and sounds from the ocean.
* The list includes:
1. **A massive, unidentified sound** detected by NOAA, possibly from an enormous, unseen structure (> stadium-sized).
2. **An unidentifiable, massive sea creature** (possibly a whale-like animal) captured on video at extreme depth.
3. **The "Slow Down" sound**: a mysterious, low-frequency moan from an incredible depth, picked up by radar.
4. **Iceland's "Giant Worm"**: footage of an unconfirmed, strange creature in Icelandic canals.
5. **The Antikythera Mechanism**: an ancient, submerged Greek artifact that challenges historical understanding.
6. **The "Train" sound**: an unexplained, engine-like noise from the deepest marine trench near Antarctica.
7. **A massive, female Great White Shark**: one of the largest recorded, with a powerful bite force.
Here are the key facts extracted from the text, each given a number and kept as short sentences, excluding opinions:
1. **Less than 5% of the world's seas have been explored** (according to the US Ocean Department).
2. **Thousands of unknown species are believed to exist in the ocean's depths**.
3. **A female great white shark is the largest currently recorded, measuring over 6 meters**.
4. **The "Train" sound originates from the deepest marine trench in Antarctica**.
5. **The "Train" sound was initially thought to be an iceberg breaking but was later ruled out**.
6. **Scientists couldn't identify the source of the "Train" sound, describing it as similar to a very large engine**.
7. **The Antikythera Mechanism was discovered in a submerged ancient Greek city off the island of Antikythera**.
8. **The Antikythera Mechanism is approximately 13,000 years old and has been underwater for thousands of years**.
9. **The purpose of the Antikythera Mechanism is still debated among historians and scientists** (Note: While the text implies mystery, the fact itself is about the existence and age of the mechanism).
10. **Footage exists of an unidentified, large creature in the canals of Iceland, described as possibly a giant worm or strange creature**.
11. **The "Slow Down" sound was captured from an incredible depth and requires an enormous source to be detected hundreds of kilometers away**.
12. **The size of the source of the "Slow Down" sound is estimated to be over 50 meters to produce such an echo**.
13. **A video shows an unidentified, huge creature (possibly a whale) at a depth where sunlight does not reach**.
14. **The NOAA (National Oceanic and Atmospheric Administration) captured the most mysterious sound on the list, described as unlike any known marine creature or natural phenomenon**.
15. **The source of the NOAA's captured sound is estimated to be at least several stadiums in diameter, at a depth where light does not reach**.