Chasms are large trenches with walls made entirely of Ebonstone. They appear commonly in Corrupted areas. The depth of chasms vary greatly; they can be very shallow or very deep. Chasms commonly have Demon Altars and Shadow Orbs at the bottom. But, quite often, there are many shallow chasms and one very deep chasm. The very deep chasm leads to a corridor, along which, demonite and shadow orbs are found. They are often accompanied by chests and pools of water. This makes chasms a great way for low level players to find good gear and get the musket, which is gained from smashing the first shadow orb. However some chasms can be dangerous, as they are deeper than they seem from the top.
Getting down the chasm often involves a long drop and having a Grappling Hook, Ivy Whip, Cloud in a Bottle, Lucky Horseshoe, or Wings can help with the fall. Dropping torches down the chasm using the "Drop item" key (T is default) will help identify paths down.
Shadow Orbs are usually encased in a circular shell of Ebonstone. To make your way to a Shadow Orb, you have to use Dynamite, (or as of 1.1, bombs ,) to destroy the Ebonstone blocking the way, Purification Powder to change the Ebonstone to regular Stone, or use a Nightmare or Molten Pickaxe or any type of Drill, since Ebonstone cannot be mined with any other lower tier pickaxes.
Getting out of the chasm is easier when using a Grappling Hook/Ivy Whip or by placing solid blocks under oneself when reaching the apex of a jump. A more permanent method of getting up and down Chasms is by building a series of Wood Platforms up one side, allowing one to climb and descend easily. A Magic Mirror would work as well, though you would have to start all the way back at your spawn point. To mitigate this, you can build a small house just outside of the chasm and set your spawn point there by using a Bed. A very quick way of getting out of a chasm would be to use a gravitation potion. The easiest way to get out is to log off the world and then log in; you'll be at your spawn.
There are occurrences of chasms in a Corruption not going deep enough to reach the point that connects them, as shown in the gallery. Sometimes, the Corruption may only spawn a single chasm, also shown in the picture. Also, the chasm may become Hallowed when the Wall of Flesh is defeated, in which case the Ebonstone blocks and walls will change into Pearlstone.