Rick said:
Please define the properties of the Stargate. 2-way travel, 1-way travel, connection required?
Obviously, if it's open all the time, the Martian end will be in deeper water (due to gravity/pressure differences) but you'd still have a 'cold spot' in the shallow water which would have been noticed where you have put it - needs to be more remote, and near a polar latitude (to near equalise the Martian water temperature) if possible.
Due to submarine activity during WW2 and the cold war, most areas approaching the US port's have been mapped, re-mapped and are monitored closely, it's simply not feasible that something that big wouldn't have been noticed before. You need to get a bit creative with your location - may I suggest somewhere that had a glacier covering it until recently, until it retreated due to a warmer climate; might even be in a lake or big river ("Scientists baffled by saltwater inland lake"), or somewhere coastal, but more remote - Alaska perhaps ("Oil survey finds mysterious object").
I was thinking of having it discovered in the 19th century when they were laying out telephone cable across the Atlantic, but I wanted to leave that option open. If it was discovered earlier than today, it would have an effect on history. Lets say it was discovered in 1866 for example, you might then have a pattern of colonization that was similar to the settling of the West, you'd have frontier towns on the shores of the Northern Ocean, you'd have saloons, the railroad, telegraph wires, maybe outlaws hiding out in caves in the Valles Marineras, there would be gold and silver mines, maybe some native Martian/Humans. (They were descended from Paleo-Indians that settled North America when the ocean level was lower and the stargate was on dry land) the Martian cities predate the arrival of the Paleo-Indians, which means they are over 30,000 years old, Archeological evidence indicates they were inhabited by humans, and modern humans at that! This has a lot o 19th century scientists scratching their heads. When astronomers train their telescopes on the Martian skies, they can see the asteroids, they discover that Jupiter is closer, the Earth and its Moon is sometimes a morning star and sometimes an evening star, they can see Phobos and Deimos clearly enough, but the planet positions don't match where they think the planet positions should be as established from observing those same planets on Earth. Since they have no optical telescopes, they don't notice the radio silence from Earth. the 19th century scientists don't have a theory on how the stargate works, thus they refer to I simply as a "teleporter".
New towns are established on Mars, immigrants come from Europe hoping to make a new start on life. Events that occur in the American West are reflected also on the Martian Frontier. The Railroad goes through, gold is discovered and there is a gold rush. Various scientists pay a visit, and begin fossil hunting, they find he fossil record very scant, digging down deeper than a few feet produces no fossils at all, they are very puzzled on how life arrived on this planet. The wreckage of an ancient spacecraft is uncovered, the engine is a mystery, and its over 30,000 years old, but they find something that's written in English, it appears to be a manual of some sort. Nothing works of course, it is not clear how the spacecraft was operated, now dials or controls were found, instead there were a bunch of dark glassy surfaces, there was a stick, supposedly a control stick, but no instrument panels, just these dark rectangles of glass. Microscopes revealed tiny little wires about the size of bacteria, engineers are puzzled about the fabrication techniques in making something so small!
Later on the first Model T Fords are seen puttering across the highways of Mars, there is drilling for oil and gas, but no crude oil deposits are found! The search in on for deposits of coal again no luck! the Nation goes to war, the Germans are defeated, and its the 1920s, and then the Great Stock Market Crash of 1929, and a Depression ensues, more people seek their fortunes on the Martian Frontier after the Great Dustbowl in Oklahoma.
To answer your question, the stargates are two-way and are always open and have been so over the last 30,000 years.
