A nearly 100-year-old building in downtown Searsport, set very close to the busy coastal road, is something of a local landmark. But it was constructed on pillars set over a drainage stream, and every time there is an extreme rain event, water floods the building. That’s why it’s about to be torn down. The demolition of the Hamilton Learning Center building on Route 1 is set to begin this week, according to its owner, the Penobscot Marine Museum. Still, making the call to have it taken down wasn’t easy, according to Karen Smith, the museum’s executive director. “It was a careful, thoughtful decision that we’ve been working on for years,” she said Monday.