The King Fahd Causeway is the only road connection between Saudi Arabia and Bahrain — a 25 km combination of bridges and earth embankments crossing the shallow turquoise water of the Gulf of Bahrain. It opened in 1986 and is heavily used by Saudis going to Bahrain for the weekend and by Bahrainis commuting in the other direction. Roughly 18 million crossings happen each year.
For visitors, the causeway is at its most interesting as a halfway-point excursion: the artificial Passport Island in the middle, where the two countries' border posts sit back to back, has an observation tower with a restaurant and a small mosque. Both passport halls offer panoramic views of the structure and the sea. A weekday morning crossing avoids the long queues that build up on Thursday evenings and Saturday nights.