Caves were implemented on the Underground Update, allowing the player to explore the dark caverns that lie beneath the islands. It is always good to be prepared, and you really want to be prepared when exploring the caves.
Although time passes, there is little indication about the actual time while inside a cave. You can keep track of how many days pass by listening to the music. Although the clock doesn't show, you still need to wait for nighttime to sleep in tents (to regain sanity). Additionally, day/dusk/night cycles can be determined by what mushrooms are up and harvestable plus, if the bunnymen are in or out of their hutches. If the sun is up, walk under a 'light hole' and you can see the clock when there.
Unless playing Reign of Giants, there is no such thing as weather in the caves or ruins, therefore some players like to hide in caves during the winter. If you do go down in the winter, it might be a good idea to leave your winter clothing at the exit since you will need them when you get out.
In Reign of Giants, outside weather and temperature will affect conditions in the caves and ruins, albeit to a lesser extent. Rain will hydrate plants and slowly increase a player's wetness. Heat will cause crops to wither (but not catch fire) and eventually overheat the player. Cold will eventually freeze the player.
Caves are underground, so natural light is limited. While it may seem like a reasonable assumption that standard lighting rules no longer apply while spelunking, it is not. Charlie will still kill you, so you must prepare yourself to always be with some form of light.
The best light source before you start spelunking is a Miner Hat; as it will allow you to use your tools and see much further than a torch. However, with the addition of Lanterns you may not need anything more than a torch to find the first Light Bulbs required for your first Lantern - It should be noted this will require an Alchemy Engine to prototype and therefore you should have one already done to avoid a trip back to your base camp (given the huge light radius granted by the Lantern this is generally perfectly justified). Once you have a Lantern most of your lighting problems are solved, simply place the Lantern on the ground should you require the use of your hands. It may also be worth crafting a second lantern in case of an emergency where you can't pick up a placed Lantern due to monsters.
Keep in mind that while you can turn placed Lanterns off to conserve fuel, if you don't have sufficient light to see them again you will NOT be able to turn them back ON again nor pick them up even if you know their exact position!
Always remember you can refuel your Lantern using Light Bulbs or Slurtle Slime, the latter being preferable given the current limited uses for Light Bulbs. Keep in mind that Light Bulbs perish in 6 days so building up a stockpile even with an Ice Box can be risky. Given their fast regrowth rate, it is better to only harvest a few at a time.
Armor and weapons
Spelunking is a late game activity. Therefore if you are not at ease, you should have nothing less than a log suit equipped, and have one in your inventory as backup. If you have a lantern and a helmet, wearing a backpack should be fine.
Tentacle spikes are good weapons since they are relatively easily to obtain in marshes. Once you get the hang of caves, it is advisable to carry a few of these. If you want to save room in your inventory, a ham bat can be used and will give you a good indication of time.
Sanity can be regained by sleeping in a tent or eating Taffy. Taffy can only restore 15 sanity so you will have to eat 3 to restore 1 days worth of spelunking. Bear in mind that taffy will rot so this is only an option for a few days (as the sanity bonus doesn't apply when the taffy becomes stale). If you do plan on sleeping a lot, make sure you have a food supply as sleeping reduces hunger significantly. Be aware that some characters may refuse to sleep underground claiming that it is too dangerous. The Tam O'Shanter will restore 6.7 sanity/min against the -5 sanity/min for being in a cave, so this along with a sewing kit will allow a long term cave-dwelling option. However, you will need to carry or place a Lantern as you will not be able to use the Miner's Hat while wearing the Tam O'Shanter. If the Tam O'Shanter has not been acquired, a Top Hat could be used (3.3 sanity/min) with occasional sleeping.
Flowers and bees are a surprising solution to the issue of cave sanity. Homeless bees produce a new flower after pollinating six flowers so carrying at least 6 live butterflies into the caves, planting them as flowers and then releasing at least four or five bees will quickly produce excess flowers that can then be picked to restore 5 sanity each. As long as you've planted the flowers in an area that you visit frequently enough, the bees will continue to create new flowers all year round because it is permanently summer in the caves.
If you have a source of edible meat such as a pond for frogs and fish, then build drying racks to make jerky. Batilisk wings can also be dried to provide small jerky. As long as there are no Bunnymen awake nearby, you can safely harvest and eat the jerky. Small jerky will restore 10 sanity, while large jerky restores 15.
Underground mushrooms are a valuable but non-renewable source of sanity, especially if followed by other food to negate the slight health loss. Mushrooms do not currently seem to respawn after they've been picked in caves because there is no rain underground, so you have nothing to lose by digging them up. A cooked green mushrooms will restore 15 sanity with the loss of 1 health point, while a cooked blue mushroom restore 10 sanity at the loss of 3 health points.
In Reign of Giants, it will still rain in caves, so mushrooms will respawn.
Keeping yourself fed will be an issue in caves, more so than above ground. As this usually is a late-game activity, a base camp with a decent sized farm/method for obtaining food should already be in place above ground. It may be advisable to stock up on jerky and take it down unless there are Bunnymen close to your entrance point. Additionally, it is a good idea to build a small base camp that can be used, especially if you're planning on staying in the caves for a long period of time, so, building a crockpot is a good idea, along with a jerky rack for keeping meat. Although be aware that Depth Worms are particularly attracted to areas containing built items, so you may need to build Rabbit Hutches to defend your camp.
For meals, meat (from bunnymen), monster meat or batilisk wing will be your main options as well as a number of carrots and berries (usually in a small area) so making meatballs or meaty stew is probably best (if you have a jerky rack, dry some of the meat, use the rest with 3 fillers to maximize your food output and keep a continued supply).
Farms work underground, although the plants do tend to take longer to grow. This is especially true if they are in complete darkness, so try to situate farms in an area that gets a shaft of daylight if possible. Building Farms in fields of Light Flowers can accelerate growth and ease harvesting. As guano is generally plentiful underground, you can force your vegetables to grow faster by fertlizing them with it. Rot and manure will also work but you need more of them. And there are no birds down here, so you will need to bring a supply of seeds with you. Alternatively, building a Birdcage and importing a bird to allow renewable farming. Building a Pig House will allow the proceeds to be converted into Manure for more farms. Dragonpie is useful for the healing properties, and will not anger Bunnymen. Carrots are also useful to farm, as they can be used to befriend Bunnymen and construct Hutches. If the Bunnymen are used for Depths Worm protection, the dropped Carrots (2 per Bunnyman) can be used to amass a steadily larger army, provided that there are nearby Hutches. Care must be taken to avoid gathering the Meats or Monster Meats while Bunnymen are around, however Carrots and Glow Berries can be looted with impunity.
If you plan on using a tent or sleeping to regain sanity, then you should double up your meal requirements as it is very easy to eat through a stack of food with the hunger penalties when you sleep.
Depths Worms are one of the biggest problems in the caves. If you see a tempting, glowing blue flower, do not try to pick it!
Depths Worms are hard to take on alone as they have 900 health and a bite from one causes 75 damage. They also chase you a surprisingly long way and there's often more than one of them. Tooth traps are ineffective because they don't trigger until the worms surface and you won't have time to reset them before the worm attacks again, but bee mines work well. It is possible to dodge them if you keep moving and you can outrun them but this makes it hard to do things underground, particularly since they are attracted to areas containing harvestable items such as light bulbs or berry bushes. Built items also attract them, so they will tend to spawn in the middle of your camp or near the exit of the cave. The best defense is to build a large number of Rabbit Hutches in areas known to have Depth Worms because the Bunnymen will attack them on sight.
In Don't Starve Together, Depths Worms remain as your worst nightmare if you try to spelunk after day 100. Since the Ruins and Cave are now on the same level, you will likely encounter up to 3 Depths Worms at the same time around the entrance to the ruins, where there seems to be a high probability for the Depths Worms, Monkeys, Slurpers and Spiders to stack close to each other, thus easily overwhelming any unprepared players, especially during a Nightmare Phase. So take extra care while in DST, where you start to see a lot of Lichens.
Caves come with their own set of spiders: Cave Spiders. These will act rather like slurtles and snurtles in the sense that they will retreat into their shells when attacked. They spawn in spilagmites, along with spitters, another variation which shoot webs at the player, dealing a fair amount of damage. They have no shell and are fairly easy to kill. Their nests can be destroyed with a pickaxe.
Slurtles & Snurtles
When attacked, they retreat into their shells. Wait for them to stick their necks out; attacking the shell directly takes quite a bit of time. In addition, broken shells drop as fragments rather than a whole shell in most circumstances.
Another option is to place large piles of minerals near their mounds. After an earthquake they will emerge to eat the minerals and they will often fight each other over the resources. As long as you aren't carrying any minerals, you will be able to safely collect any slime, shelmets or armor that they drop. Even if they don't fight, a large pile of minerals will ensure a good supply of slime as they sometimes excrete it after eating
Another option is to burn them! this will destroy the slurtle very fast, but the pick up items are destroyed too!
Bunnymen are hunchback mobs and hostile only if you're carrying meat. It is therefore recommended to spelunk with vegetables and fruits. In particular, carrots are noteworthy as they cause bunnymen to become short term allies. They will also try to run away if they are at low health, making them harder to kill.
You can also keep meat items inside chester.
Bunnymen are easy to kill if you hire a few Rock Lobsters. When the hired Rock Lobsters attack the Bunnymen, other Bunnymen will be compelled to attack the Rock Lobsters allowing you to deconstruct their hutches and obtain meat in the process.
That said, bunnymen make useful allies because of their fast movement speed if you can manage to obtain the carrots to hire them. They are also an efficient defense against Depth Worms because they attack them on sight.
If playing as Webber, Bunnymen will attack you on sight, regardless if you have meat in your inventory or not.
Rock Lobsters make great allies when trying to clear paths in the caves. Hiring 4-5 at a time is recommended since they hide in their shells when attacked making them rather weak in 1 on 1 fights. Due to their slow movement speed, the player must kite enemies towards the Rock Lobsters at the beginning of the battle.
Batilisks are strange mobs, as they behave differently at different times. Sometimes they will fly directly at the player, while other times they will pointlessly try to move through a wall. Like spiders, batilisks will swarm you if you attack one, and if you stand still they will circle you. Sometimes, they will attack, while other times they will circle you with no apparent purpose but to annoy you. Be wary of these mobs, as they wander. They have no "home" that they return to (unless they are on the surface, in which case they return to the sinkhole they came from during daytime.) They are worth farming, as they have decent drops and periodically drop Guano, which is better for fertilizing crops than manure.
Killing batilisks can be difficult, as they are very small and very fast, but it is recommended to carry a spear and logsuit: you can kill them in two hits with a spear (one, while playing as Wolfgang in his mighty form, or Wigfrid using a Battle Spear), and will only take 1 HP of damage per hit from the batilisks. A Tentacle Spike will kill them in one hit and you can often avoid being hit yourself as long as you attack them first. Due to their tiny size, it is also recommended to press or hold the 'F' key while near one, rather than hopelessly attempting to click on it. However, if the player is playing the console version of Don't Starve, no extra buttons have to be pressed, only the attack button. When playing as Wigfrid, it may be wise to set up base (in Sandbox mode) near an unplugged sinkhole, as the commonplace of the wings could be useful.
If you feel like any more tips should be added, add them right away.
Sinkholes are the entrance to caves, usually found somewhat near graveyards and the pig king. There are three sinkholes located in the world. A good way to spot them is to look for the rocks on the ground which always mark the presence of one in your vicinity. They can be mined with a pickaxe to gain access to the cave but beware; at dusk batilisks and occasionally other mobs will surface and roam during dusk/night. Therefore, it is not advised to set up a base near one. When exiting on to the surface, the hound warning will play, and the hounds will soon come. It is advised to build a hound bunker to avoid this. A firepit and winter clothing will be needed if you come out in winter, otherwise the player will freeze quickly, and coupled with the hounds, may overwhelm the player.