Chasms

Chasms are deep tunnels found in The Corruption or The Crimson. They generally begin at surface openings, and are lined with a thick layer of Ebonstone/Crimstone, respectively. They tend to reach depths around where the Cavern layers begin. Large clusters of Corruption/Crimson enemies tend to spawn and collect in these cavities.

Corruption Chasm
A Chasm in the Corruption is a long vertical pit beginning at the surface, dropping straight down into a dark abyss. The walls are made of Ebonstone, with Ebonstone Wall backgrounds. An intersecting horizontal tunnel generally connects Chasms near their bottoms, and contains several Demon Altars. Shadow Orbs tend to appear below Chasm floors, buried some distance beneath. They are usually enclosed in Ebonstone, rendering them inaccessible without mid-tier tools or explosives, such as dynamite or bombs.

Crimson Chasm
A Crimson Chasm is lined with Crimstone, and begins at a more slanted entrance, rather than a vertical pit. Crimson chasms are less linear, and can branch or zigzag. They tend to lead to large chambers that appear to have the shape of hands, and branch out into veins, each of which lead to Crimson Hearts; These are usually enclosed in Crimstone, rendering them inaccessible without mid-tier tools or some form of Explosives capable of destroying tiles. Crimson Altars can be found concentrated at a single layer on the descent into the chasm.

Corruption

 * Players without the ability to mine Ebonstone and who have not yet obtained a climbing aid like Grappling Hooks will want to avoid falling into chasms. Some chasms can be very difficult or impossible to climb out of by merely jumping between ledges.
 * Beginning players may want to cover corruption chasm entrances with Blocks or Platforms to avoid inadvertently falling in when traversing the surface.
 * Running Platforms across the shaft at intervals can help, as can running Ropes down the length of the shaft.
 * Using Purification Powder on the Ebonstone will turn it into Stone, which you can mine with any Pickaxe. This is useful for getting to the Shadow Orbs if you don't have access to explosives.

Crimson

 * As Crimson chasms tend to be slanted rather than vertical, they are usually easier than Corruption chasms for beginning players to navigate without climbing aids. It is still possible, though less likely, to suffer fatal fall damage or become trapped in Crimson chasms until death, in which case the player can save and exit the world if no such tools, such as ropes or platforms, are available.
 * Chambers containing Crimson Hearts are frequently flooded with water. In such cases the player may need a water-breathing item, such as Gills Potion and/or Flipper Potion, to reach them safely.
 * Similar to the corruption counterpart, Crimstone can be returned to the purified state with Purification Powder, allowing access to the Crimson Hearts.