Possible Clues:

• Percival Lowell "discovery"
• Is a half-century enough to be able to get it dug?
• Structure with locks
• That's dug by way of water
• Body of water, lake beneath miraculous site
• Ottawa Region's Rideau______
• Man-made channel of water
• Is a half-century able to have made way for water?
• Place for a low bridge
• Martian feature