The Skull Cavern is an area accessible in the north-west corner of Calico Desert.
To gain access, the player must find the Skull Key, which can be obtained from the lowest level of The Mines. Mr. Qi will reward the player for reaching level 25.
Monsters in this mine are a lot more powerful, and exploration is made more difficult by the fact that, unlike the mine near the town, you don't unlock fast-travel points using an elevator. The probability of getting rare drops is much higher here compared to normal mine in the town. In this cavern, every time you start, you will always start from the first floor. Bring lots of healing items if you intend to explore it! There is no end to the Skull Cavern.
The easiest way to get a lot of Iridium ore or rare drop is by descending into the lower level as fast as you can. The deeper you go, the greater the loot you will get. Bringing Mega Bombs will make the job easier for clearing out those obstacles like rock, making it more efficient rather than using Pickaxe which is more time-consuming. Jumping down the hole rather than using stairs will also do the trick but is quite risky since you will get a certain damage when you are using the hole depending on how much floors are being skipped.
Remember to bail out when your food runs out or if it's getting nighttime. If you fainted in the cavern, you will be losing plenty of money (1000G) and several items, including your weapon, so be careful.