Locked chest

Locked Chests (also known as Steve Co. Supply Crates) have been added with Minecraft Beta 1.4. Clicking on the chest will bring you in the screen shown on the bottom. Clicking on "Go to Store" will forward you to the Minecraft Store. The chests emit light at level 15 and are indestructible by hand or with a tool, but can be destroyed with 1 piece of TNT. They are an early April Fools Day joke.

Trivia

 * Locked chests are a reference to the Mann Co. Supply Crates from Team Fortress 2, where a key must be purchased for $2.49 in the in-game shop called Mann Co. Store in order to open the crate.
 * Locked chests are indestructible by hand, but can be destroyed with one piece of TNT.
 * When you go on the Minecraft store website and click proceed to checkout, a screamer occurs when a dinosaur jumps out of the bottom of the page, making a growling noise and flying off the screen showing that this is indeed an April Fool's Joke.
 * Entering items totalling over 10,000 Dollars into the cart causes a window popup asking if you are prone to seizures. Entering OK makes the screen start flashing. Earlier this was released there was no seizure warning label, Eventually notch implemented it to avoid lawsuits and posts on the forum.
 * This may have been what Notch was talking about in his Twitter post about a "silly secret project".