a very reactive metal

Aluminium is a light and extremely strong metal. It is used in aircraft, boats and in household cooking pots. What most people do not realise is that aluminium reacts vigorously with water to produce explosive hydrogen gas. The reason why aluminium does not react so vigorously in Nature is due to its impenetrable rust layer. Unlike iron whose rust layer flakes off to expose more iron, aluminium oxide (aluminium rust) stays firmly attached to the surface of the metal preventing oxygen from penetrating deeper into the metal. If the rust layer is scraped off it quickly reforms protecting the metal underneath. This is why we can scratch, drill and bend aluminium without it reacting with water to produce hydrogen gas.
Once the aluminium oxide layer is removed, rapid oxidation occurs with the water producing heat and hydrogen gas. The picture on the right shows aluminium foil in 1M NaOH solution. NaOH removes the rust layer and prevents its formation thus exposing aluminium to the water. Notice the production of hydrogen gas.
Click to see a 120kb movie of the reaction of aluminium and water.
The reaction can be written as
2Al(s) + 6H2O(l) => 3H2(g) + 2Al(OH)3(s)