What causes tidal waves? Also, how large can they become and what are the chances that they will occur?
First, a linguistic note. "Tidal wave" is a perfectly legitimate English term referring to giant destructive waves; it does not imply that they're caused by the daily, moon-related tides. However, some consider the phrase to be misleading, if not outright inaccurate. As a result, the preferred practice among oceanographers and pedants is to borrow the Japanese term, tsunami, which means "harbor wave." (This is not objected to on grounds that the phenomenon is unrelated to harbors, because most speakers of English don't know Japanese. That a non-literal foreign coinage is more acceptable than a home-grown one is one of the language's little ironies.)
Tsunamis are the result of a sudden disturbance in sea levels in a short period of time. They can be caused by underwater earthquakes, volcanic eruptions, meteor impacts, landslides beneath the ocean surface, or other disturbances along those lines. It should be noted, however, that not all underwater earthquakes or similar disturbances cause tsunamis.
Tsunamis may reach speeds of 500 mph (800 km per hour) and when one approaches shallow water along a coast, they are slowed, causing their length to shorten and their height to rise, sometimes exceeding heights of 100 ft (30 m).
What are the chances one will occur? That depends on where you are, but tsunamis have occurred in all the major oceans. They are most common in the Pacific Ocean, which has been given the nickname "ring of fire" for being the most geologically active area on the planet. On the other hand, frequency and devastation are two seperate things: the most devastating tsunami on record, at the end of 2004, took place in the Indian Ocean.