Sand Bridge At Low Tide

An apparently inaccessible island has a secret access -- a bridge formed by a high sand bar that is revealed only at low tide -- and sometimes only at night as well.

Not only does this allow an island to be a little accessed secret, but also provides the opportunity for people to be trapped on an island while plot happens at them.

Anime and Manga

 * After all the trouble Wataru goes through to get off of Promised Island in Sister Princess, he discovers that he could have walked away on the one night a month when the dry land path connecting it to the Japanese shore appears.
 * In the Festival Episode of Love Hina, Sarah and Su strand themselves on an island, where they are soon joined by a "shipwrecked" Kitsune. After a night of worrying how to get themselves rescued, help comes to them across a bridge of sand revealed by the tide.
 * The same happened in the Beach Episode, but to Naru, Keitaro, and Sarah. They spend the day on the island, until nightfall, when the tide lowers and they're able to walk across and make their way home.
 * In the episode of Pokémon called "The Crystal Onix", Ash and the gang find that the way to the mythical cave is a sandbar that only appears at certain times of the day.
 * In Nadia: The Secret of Blue Water, Marie discovers a sand bridge that forms at least once a day between the beach of the deserted island the children find themselves on, and the piece of the wrecked ship which is caught upon a rock just off shore.
 * In Umi no Misaki, a small island is connected to Okitsu by one of these for a few days a year. Naturally, there's a story and cape maiden ritual associated with it.

Literature

 * A Song of Ice and Fire had a number of them, such as the monastary at the Trident delta and the castle that Bran hides in.
 * An island chain in the Belgariad was used twice. Played straight once in the main series, with the path between islands revealed at low tide. Subverted in the prequels, when it's first discovered, but the protagonists skirt it in favor of inconspicuously crossing the frozen northern sea.
 * In Meredith Ann Pierce's The Woman who Loved Reindeer, one of these allows a migrating tribe to continue on their way. (Capricious spirit-beings are involved.)
 * Plays a critical part in the famous Old English poem The Battle of Maldon, a commemoration of real-life events: In 993 AD, an army of vikings had landed on an island in the Blackwater estuary in Essex which was connected to the mainland by a causeway only accessible at low tide. Englishmen and Vikings fighting over the causeway makes up the first part of the poem.

Video Games

 * Done backwards in Breath of Fire IV our heroes walk onto Saldine island, then find themselves stranded at high tide.
 * Dead or Alive: Extreme Beach Volleyball has a sandbar as a place to play volleyball.
 * Mario Kart 64's Koopa Troopa Beach uses this trope as a shortcut.
 * Double Dash! has a course with a similar gimmick.
 * The Oni Stepping Stones in Aoi Shiro, only exposed during low tide at certain times of the year.

Real Life

 * Mont Saint-Michel, a French islet in the coastline of Normandy, can be reached during the low tide by a bridge of sand.
 * Also, Saint Michael's Mount, a similar Cornish islot which can be reached by an artificial causeway, only accessible at low tide. This troper does not know why they did not just build a bridge instead, but Mount's Bay (the bay it's in, obviously) was once not-flooded, so this may have something to do with it.
 * In historical times, both were ordinary pieces of land - the change to part-time island happened rapidly in geological terms but over a number of human generations. And causeways allow water traffic without having to build a drawbridge - especially if the waterway is too wide for a drawbridge to be practical.
 * The bar that gives Bar Harbor, Maine its name stretches out to an island at low tide. The tidal changes are actually quite spectacular.
 * The Waddenzee which sits inbetween the mainland of the Netherlands and its tiny chain of islands. People frequently trek out to the islands during low tides as a family outing. This troper did it as a high-school outing.
 * Lindisfarne in Northumberland, a popular tourist destination for its historic castle and monastery. There is now a tarmacked road to the island.
 * A rather dramatic example occurred when there was significantly more water in the form of ice than in the form of, well, water, which allowed ancient people to literally walk over what is now the Bering Strait and into North America. Similar theories are offered to explain how ancient people settled islands across the Pacific.
 * The latter theory is unnecessary and superfluous. Those people have demonstrated excellent skills in boat-building and navigation; the chances are that they reached those islands by water quite deliberately.
 * Note also that we know for a fact that people were using boats because there are quite substantial oral records of islands being settled by such-and-such chief's son sailing from such-and-such island.
 * Wrightsville Beach and Shell Island in North Carolina used to be connected by one of these until someone got the brilliant idea to add more sand to the sandbar, connecting the two permanently and giving them more land to build on to boot. So imagine what happened when Hurricane Fran came through and attempted to remake the original inlet...
 * This is how Sometimes Island (an island in one of Kodiak Island's bays) got its name.
 * Bribie Island just off Queensland, Australia used to have one of these but it disappeared when they dredged the channel for navigation purposes and the island is not connected to the mainland by a bridge.
 * It is believed by some historians that the Aeta peoples, one of the indigenous people of the Philippines who currently reside in the mountainous parts of Luzon, arrived there from mainland Asia via land bridges over 30,000 years ago.
 * Charles Island in Milford, Connecticut, although a bit unfortunate in that leaving too early after low tide has the probable consequence of you getting stranded. Despite people clearly being visible from the beach nearby, this has left people stuck out there for days.