A gray-shingled [Nantucket](https://www.realtor.com/realestateandhomes-search/Nantucket_MA) residence—dubbed “ [The Shack](https://www.realtor.com/realestateandhomes-detail/37-Sheep-Pond-Rd_Nantucket_MA_02554_M42655-63917)” and in danger of eventually plunging into the ocean—received a tidal wave of interest when it hit the market recently for $850,000. The tiny Sheep Pond Road cottage on the tony Massachusetts island offers just 696 square feet, steps from the sea. And it’s the week’s most popular