For decades, scientists searched for Mars' lost ocean: A giant 'bathtub ring' may have finally revealed it

• PC: ReutersFor years, the idea that Mars once held vast amounts of water has remained one of planetary science's most persistent questions.
• Dry river valleys, mineral deposits, and ancient lake basins have all hinted that the planet looked very different billions of years ago, but proving the existence of a true ocean has been far more difficult.
• The biggest obstacle has always been the landscape itself.