You feel this article will be informative.
A "Level Feeling" is a common feature of roguelike games that add a small chance of special themes being randomly applied to levels that play a little differently to normal.
Level feelings usually add special rules to the level, or unique structures and enemies that do not normally generate.
The name comes from how the special themes are commonly announced by exposition in roguelikes, as if the character has a vague sense of premonition about what they are about to run into.
Level feelings can stack; it is possible to have, for instance, a flooded Restless Dead level, or a dark Snake Pit level and so forth.
Spelunky Guy will always say something at the beginning of a level to indicate what level feeling is active, but will pause if there is more than one feeling before mentioning it.
Terrain Features: The level will be in total darkness, except within a certain distance of light sources. Your vision in the dark is limited to a very narrow radius, and your distance from light sources also affects how dim the screen will appear.
The game helpfully generates a Box of Flares near the level entrance which is the most useful portable light source available. There will also be other light sources generated around the terrain that do not normally appear, such as lanterns. Additionally, traps such as Arrow Traps and Tiki Traps have candles on them that generate a small amount of light. Certain weapons and equipment, such as guns and bombs, generate a brief but bright flash of light when used.
Unique Enemies: Scarab
Notes: There will never be more than one dark level per area.
Additionally, dark levels will not appear if you completed the previous level in 20 seconds or less, in order to prevent frustrating speed runs.
"I hear snakes... I hate snakes!"
Location: The Mines
Terrain Features: The level contains a snake pit, a long vertical shaft (with cubbyholes along the sides) filled with Snake. The large amount of treasure that spawns here generally makes a snake pit worthwhile to explore, provided you have enough rope or bombs to facilitate escape afterwards - two or three ropes is usually a safe amount. A snake pit always has two rubies in the bottom of the pit guarded by five snakes. Falling from the top of the snake pit is usually fatal as the snakes will attack you while you're stunned.
A mattock can be found directly underground at the bottom of the snake pit, retrievable by the use of a bomb; with a little luck, this can allow you to get through the level using only a single bomb.
Unique Enemies: None, but snakes are found at the bottom of the pit in a far higher concentration than they can usually spawn.
Trivia: The phrase "I hate snakes!" may be a reference to Indiana Jones, who is known for disliking snakes.
"The dead are restless!"
Location: The Jungle
Terrain Features: Mostly normal, except Gravestones will generate on top of the terrain which can be destroyed for loot. More bones and Skeletons will be found than usual, trees will appear to be dead, and piranhas will be skeletal. There will be no Golden Idol on this level, but instead there is a chance of finding a Crystal Skull. Once the Crystal Skull is taken The Ghost will appear suddenly afterwards.
While most graves in this level are labeled "RIP," there is a chance (but no guarantee) of finding one labeled "ASH." If this grave is exploded, inside there is a shotgun to be found.
"You hear rushing water..."
Location: The Jungle
Terrain Features: A large portion of the bottom of the map is one huge lake. The exit tunnel is always found above the water level on an island.
Unique Enemies: Old Bitey appears in the large lake at the bottom of the map, along with a whole shoal of regular Piranhas.
The Black Market
"Welcome to the Black Market!"
Location: The Jungle
Terrain Features: The upper levels of the area are comprised of six shop in rows linked by ladders, and one shop unique to this area on the far right edge that sells the Ankh. The bottom layer is ordinary terrain.
Unique Enemies: None, but is populated with all the normal hazards in addition to six or seven Shopkeepers with itchy trigger fingers. (Or, if you're wanted, make that eight.)
Notes: This level is not applied randomly at the start of a floor, but the secret entrance must be located in area 2 behind solid terrain using the Udjat Eye and revealed with a bomb or a mattock.
"It smells like wet fur in here!"
Location: The Ice Caves
Terrain Features: Unlike ordinary levels in the Ice Caves, the terrain of the Yeti Kingdom is much more continuous (although a large portion of it is ice), so the risk of falling to your death is not as great. However, fragile ice bridges with spikes underneath are common, and the loose blocks dropped by the Yeti King can block passages or crush you unexpectedly.
UFO Crash Site
"There's a psychic presence here!"
Location: The Ice Caves
Terrain Features: Most of the space in this level is just empty caverns, with a large alien Mothership embedded in one of the walls. A jetpack can always be found hidden in the wall behind the Alien Lord.
"I can hear prayers to Kali!"
Location: The Temple
Terrain Features: A deep pit spawns with numerous Arrow Traps and a pool of lava at the bottom. A Damsel and a Golden Idol can be found suspended above on a platform. Picking up the Idol causes the platform to disintegrate instantly, sending the Spelunker into the pit. This usually results in death, unless he has climbing gloves or a Jetpack to aid his escape.
The ground tiles underneath the lava pit contain Rubies.
Unique Enemies: None, but a Damsel is guaranteed to be found.
Notes: There is a glitch involved with this feeling as of version 0.99, due to the limit of only one Damsel being allowed to spawn per level. If this feeling occurs on a level with a Kissing Parlor, the Damsel will spawn above the pit as usual, but the shop will claim her as property and picking her up will brand you a thief.
The City of Gold
Terrain Features: The area is not too different from a regular area 4 level, except all the solid tiles in the room are made of pure gold, which drop gold bars when broken. Additionally, there is a unique Golden Statue which can be broken open for Diamonds and other assorted jewels.
Unique Enemies: None, but more Mummies spawn than usual.
Notes: This level is the most complicated and difficult location in the game to reach, given the dangerous and tricky quest of unlocking the Big Chest in area 1, using the Udjat Eye found within to locate the Black Market in area 2, purchasing the Ankh for $50,000, sacrificing your life in front of the Moai in area 3 to acquire the Hedjet, defeating the Mummy in area 4, and bringing the Scepter it drops to the golden door all in a single playthrough.
Anybody who manages to reach the City of Gold without cheating deserves congratulations. Anybody who manages to make it back to the surface with the loot deserves recognition!
Level feelings can be an exploited in roguelikes like Spelunky with a technique known as 'level scumming'. This is done by generating a new level in the hopes of being given a certain feeling, and quitting and retrying if the feeling does not appear.
For instance, if the player wanted to start a game with an easy Jetpack, they would take the shortcut to Level 9 in the hopes of walking into a 'UFO Crash Site' level. If it turned out to be a regular level (or a 'Yeti Kingdom' level), they would take the easy way out and go through the shortcut again.
Scumming can also be done in the hopes of finding a shop, but since shopping sprees typically require a few levels of treasure to fund, this is not a very productive strategy for those not wishing to become the target of unrelenting geriatric vengeance.