The Skull Cavern & Getting Iridium
Using the Skull Key and Exploring Stardew Valley's Combat-heavy Mine
Stardew Valley's mines only take you so far. Eventually, you'll want Iridium to upgrade your Tools and improve their power. It's the single most effective way to accomplish that until year 3 (assume you do good on Grandpa's evaluation). The Skull Cavern is a dangerous area and features new enemies and different mechanics than the mines. Here's how to get there using your Skull Key and what you'll find inside the cavern.
Get the Skull Key and Desert Access
Unlocking the Skull Cavern means reaching Mine level 120 and obtaining the Skull Key from the chest that you will find on that level. If you've got there and unlocked the chest, yet you cannot find your key do not fret - it's in your inventory in the 'wallet' section and permanently in your character's possession. The door to the Skull Cavern will open for you, but you first have to be able to travel to the desert.
Look to this guide for information on Calico Desert. Getting there requires you to complete the vault bundles that are inside the Community Center. Making the vault available means finishing 4 bundles (individual, not rooms) so that the vault unlocks. You then need a total of 42,500 gold to earn the bus repair reward by purchasing all the bundles. These are the only bundles that require only money. Of course, you can also do this with money if you chose to side with Jojo Mart. Once that's done, you can board the bus at the stop to the east of your farm a little after 10AM each morning.
How it Works: Finding Holes and Staircases
Do read up on the process of exploring the Mines. It is very similar but without elevators that allow you to return to the previous floor. Every run is an endurance run, and you start with a few hours less than when you go to the mines because of the bus schedule. You'll smash rocks, find things, look for ore and combat the enemies that assail you as you try to go deeper. When you return, you'll start back at level 1 and try to make it deeper, where the iridium is found, and repeat the process with new random levels.
Finding holes and staircases by breaking rocks while picking up anything useful along the way is your main goal. Jumping in a hole will make you take some health damage based upon how many levels you drop but give you multiple floors of progress, closer to Iridium. The damage from a fall is never a major problem if you're prepared and brought decent food, just do not jump in a hole with less than 20% health. Heal up first. Just a few levels is not much damage but a fall of 8 floors can hurt quite a bit, and it's entirely random how far you'll fall.
While the mines are safer, Skull Cavern can provide more raw mining skill experience at random. You're more likely to pass up copper and other things in an effort to go deeper and there is more combat. Thus is also provides far more combat experience so that you can master that skill, but with more risk and use of supplies. If you want level 10 mining, you may be better off starting a regular mine run from around level 100 as it's safer and more carefree.
Skull Cavern: Iridium and Enemy Mechanics
Every level you pass makes the likelihood of good ore (namely Iridium) more likely, primarily at 10-level intervals. Most of the rocks won't drop anything at all, though you have a chance at omni geodes and other minerals. More enemies will also appear, so it gradually grows more dangerous. Some levels can be particularly challenging if you drop through a hole and can't back out. It can be smart to bring enough stone to craft at least one staircase with you in case this happens, as you can place it and go to the next level to escape a fight. Some levels you'll really want to nope out of - it's entirely random and some are too dangerous. Remember that enemies have an aggression radius and try not to 'pull' too many at once by running around without that in mind.
Lucky Days and Foods for Luck
Going on a lucky day (determined by watching the fortune teller) is preferable to a neutral or unlucky one (almost not worth visiting without foods). This increases both the odds of finding staircases and holes, in addition to valuables like Iridium! You can increase your character's hidden luck stat by having a Lucky Lunch which you learn to make from the cooking channel on the last day of Spring in year 2. It requires Sea Cucumber, Blue Jazz, Tortilla (made from corn and learned from the cooking channel 7th of Fall year 1) . Farming these items you can make a batch of lucky lunches for your runs in the Skull Cavern.
Pumpkin Soup is another option that is easier to make but requires you to befriend Robin at 7 hearts. Pumpkin Soup provides +2 Defense and +2 Luck for around 8 minutes and is made with just Pumpkin and Milk which are both readily available. Farm pumpkin in large quantities during fall. Enemies in the mine will drop Spicy Eel, which you can eat to give yourself a luck and speed boost. Just do not eat more than one food at a time or you'll overwrite the previous buff. At the very least drink coffee bought at the pub for a speed boost. If you don't have access to these foods, just be sure and take advantage of lucky days with a coffee buzz and you'll still find plenty of Iridium when you travel deeper in the mine.
If you have an Obsidian Edge (gained at level 90 in the mines), you have enough firepower to progress in the Skull Cavern. However, it's recommended you save up 25,000G and go to the Adventurer's Guild. After reaching level 120 you are able to purchase a Lava Katana that does much more damage. Under level 40 in Skull Cavern you may find The Slammer, a powerful hammer, but the Lava Katana is better. Obviously you'll one day want a Galaxy Sword - the most powerful weapon in Stardew Valley - which is gained from finding a Prismatic Shard in the cavern and taking it to the pillars in Calico Desert (top right side). Serpents and Mummies have a very slight chance of dropping it as do the actual Iridium Ore nodes you'll find. The Galaxy Sword will let you dispatch the enemies you'll find here with great speed, and only mobs of them can harm you.
You'll want a nice stack of 10+ healing items to bring with you to the cavern - not bad fruit that only gives 10 health - a real meal. If you cannot cook any good foods to help heal your character, consider purchasing salads at the restaurant in large quantity. They offer the best healing to price ratio of all the foods sold there, and are plenty to keep you going. Also, they do not offer a buff so they will not overwrite the effect of a lucky lunch or pumpkin soup. You can also buy your coffee here so that you have at least one buff going into the cavern.
One big secret to moving quickly through the cavern is to utilize bombs for both progress and to kill mummies. Hopefully you stockpiled them from the mines, as many enemies drop them. Additionally, you can craft your own. The rocks in the Skull Cavern can be tough unless you've got a good pickaxe, and bombs will take out rocks in a large area after a short delay. This greatly speeds up the process of finding a staircase (OK) or hole (great) and letting you proceed to the next level. Use of bombs does grant some mining XP, but you're mainly doing this to get through barriers quickly. As noted you have 3-4 hours less than on a mine run, so it's nice to preserve time so that you can get to cavern level 30+ where Iridium is increasingly more common.
Explosive ammo for the slingshot can be handy for taking out a downed mummy, and if you have an excess you can use them just like other bombs only in a safer way. However it's expensive to make in large quantities. If you've found all four dwarf scrolls from the mines, you can buy the other three types of explosives from the dwarf (break the block to the right of the mine entrance with a steel pickaxe).
List of Enemies in the Mines
Inside the Skull Cavern you will face somewhat stronger enemies, primarily in the form of large slimes that break apart into many smaller ones (which can be very dangerous given the short period of invincibility). Here are the three main types of enemies you'll encounter that are difficult and how you can handle them.
- Armored Bugs - they patrol back and forth. They're invincible and not dangerous, though they'll hurt you if you walk into them. Sometimes breaking a rock will widen the path of a flier and make you need to avoid it more often. It's sometimes best to leave them trapped because of this.
- Slimes - There are both large and small purple slimes. Slimes in the Skull Cavern are purple and drop Iridium, so you should often kill them if it's not too far out of your way. The large ones are easy but may drop a small army of mini-slimes - up to 5. Having them swarm you is very dangerous, perhaps the worst thing you'll encounter in the cavern. The smaller slimes will jump, get knocked back and use their jump attack again. It's better to get them all against a wall, but if you can't do that you will likely want to step in between each swing if you're using a sword. This lets you reach them before the jumping animation, as you can and will be horded if you stand there swinging because their jump attack is fast.
- Serpents - These are a flying enemy and can assail you out of nowhere. Serpents are a nuisance, but I've learned that timing the swing just before they enter attack range is better than repeatedly spamming attacks. They'll usually come right in after an attack. If multiple serpents attack, try to get them all on one side of you.
- Mummies - Mummies are easy to knock down, but invulnerable once they're down and will rise again. The secret to killing them is to use a bomb of any kind (even a small cherry bomb). Crafting bombs is essential to preparing for a long run in the Skull Cavern, because of these guys and just how effective bombs are for moving downward.
In every case but the mummies (who you should hack to pieces ASAP), it's more important to TIME your swings than to chop chop as fast as you can. With the flying enemies (Serpents), unless you have a really fast weapon, you will want to swipe just as they come in to attack. That way you're not hit every time they dive in and can rely on your knockback. While with the slimes, it is better to take advantage of knockback and swipe - step in - swipe - step, and move in just a touch closer between swings. Spamming your weapon will only lock you in animation and enemies may pass through it. Especially the slimes and flyers.
Death and Preventing Lost Items
Dying in the Skull Cavern takes about 1,000G from you (unlike the mines which seems to be a portion). You'll also leave some items behind. Running in the Skull Cavern with only the necessities - a hoe, pickaxe, food, weapon and bombs, is a good idea, so that you do not lose access to other tools that may be dropped and returned via mail. Some items can be lost permanently, including weapons (but not the Galaxy Sword). Of course, you can also pass out there and will lose more gold but be 'rescued' and keep your items. You'll lose more gold that way. Always make for the ladder and bus before 1AM.
Getting Iridium Outside Skull Cavern
Iridium Ore is not only obtained in the Skull Cavern. I want to point that out first as some people will play SV to relax and don't want to mess with the combat in the cavern. It is just the most effective way to get it in bulk. Other options are buying Omni Geodes from Sandy at the Desert Oasis on Wednesdays, but you're relying on luck. Meteorites may fall on the farm (and can be mined with a gold pickaxe). One Iridium Sprinkler is sold by Krobus in the Sewer area every Friday (so this can save you some Iridium). The traveling merchant may sell sprinklers at random. I point these out as the iridium sprinkler farm you probably want end-game takes a lot of ore. Going into the Skull Cavern is the fastest, most reliable way to get Iridium in Stardew Valley.