I like the Isthmus of Rivas, in Central America, Theory the best in being the narrow neck of land as described in a Book of Mormon Geography Setting. Here is Part from that Web Site Page:
I suggest that the Isthmus of Rivas on the Pacific side of Nicaragua is the narrow neck of land referred to in the Book of Mormon. This may seem odd to most students of Book of Mormon geography as this minor isthmus does not appear to be a significant feature that would isolate Nicaragua from Costa Rica. But in fact it is. So be patient and I will try and illustrate the logic of this proposal.
The Isthmus of Rivas is a low lying strip of land between the Pacific Ocean on the west, and Lake Nicaragua on the east. On the western side the isthmus is composed of a low range of coastal mountains parallelling the Pacific coast. These hills reach a maximum height of 1800 feet. A lowlying plain, about 4-10 miles wide, and averaging 100 feet above sea level, forms a corridor bordering Lake Nicaragua. This plain is gradually narrowed by the encroaching western mountains until at the southern end it forms a narrow valley which is only several miles wide. At the narrowest point on the isthmus, south of the city of Rivas, it is about 15 miles wide. Its north-south length is approximately 100 miles. This area of Nicaragua has a wet season from May to December, and a dry season from January through April. It averages 35 inches annual precipitation, and average temperatures range from 20 degrees Celsius (68 degrees Fahrenheit) to 44 degrees Celsius (93 degrees Fahrenheit).
The vegetation consists of pasture and open grassland on the plain, with an occasional Guanacaste tree, and grass interspersed with sparse trees and brush in the hills and mountains. The area is mainly used for livestock grazing at the present time. Prior to modern cultivation the area would have been classed as tropical savannah. There is no jungle or densely vegetated forestland, at least from Rivas almost to the Costa Rican border.
Due to the topography of Nicaragua and Costa Rica, the Isthmus of Rivas has been the natural and exclusive route of travel since pre-conquest times. It presents few barriers, and provides many amenities to the traveler. All land traffic, whether north or south is naturally funneled through this isthmus. It was the only terrestrial pre-Columbian trade route, was the route of the Spanish conquistadors, and continues to be the exclusive modern route with the Panamerican Highway following its gentle course. It was also likely the exclusive route of ancient peoples during Book of Mormon times.
In close association with the Isthmus of Rivas is the adjacent Lake Nicaragua. This lake is the largest freshwater body of water in Central America, and the dominant physical feature of Nicaragua. The Indian name for the lake was Cocibolca, meaning "sweet sea". The Spanish called it Mar Dulce. It is oval in shape, has a surface area of 3,149 square miles, is 110 miles in length, and has an average width of 36 miles. It is about 60 feet deep in the center, but reaches a maximum depth of 200 feet (105 feet deeper than sea level) southeast of the island of Ometepe. Its surface is 95 feet above sea level. More than 40 rivers drain into the lake (mainly from the eastern mountains).
( http://www.mormongeography.com/ )
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