Been trying to find this book for ages since I read it around 2007-2010. I remember the plot being revolved around a middle-aged man (unsure?) trying to find a treasure that was lost for many many years.
I think the treasure was buried in the ocean or something and that's why no one could find it as during that time sonar didn't exist, maybe it was the 20th century or so. I cannot remember anything much but this is all I can recollect of this interesting story.
Maybe the guy found it as he had diving equipment with him, as turns out because climate change occurs and sea level rise the treasure was supposed to be buried in the ground but that area becomes an ocean now. The interesting part of this story is where a portal is opened at the current plot's time period (20th century) and in enters a time traveler from the future. This guy is one of those from the future who came back to the past to help the current inhabitants find the treasure, this would help the people in the traveler's time find the treasure 'faster'. This part should answer queries on whether this is Science Fiction or not!
Still remember author's last name ended with Brad as in Bradley or something the same.