A resource that you need to locate early on in Subnautica is Cave Sulfur, but beginners often don't know where to find it. The world around them is brand new, and the developers made sure to hide this away where it would be difficult to find. Cave Sulfur serves many purposes as it can help you craft a Laser Cutter, Repair Tool, and Flares.
You will need these items throughout your time on Planet 4546B as you work to make your way off its perilous surface. You might not know how to get Cave Sulfur in Subnautica, but it is easier to acquire than you think.
Head To The Safe Shallows Or Kelp Forest
The two biomes in which you will find Cave Sulfur are the Safe Shallows and the Kelp Forest. These are very safe areas, unlike some of the scarier biomes, that you can explore when you first jump into the waters of Subnautica.
If you are lost as to what these biomes look like, the Safe Shallows is where your Lifepod is located. The Kelp Forest is just outside this zone and can be identified by its tall green Creepvines that extend up from the seabed.
Locate The Caves
Once you are in one of these biomes you need to search the ground for openings that lead into a cave. They can be found on the seabed, or on the side of an underwater rockface.
It can take some searching to find them, but once you know what you are looking for you will have no issues finding them again.
Find The Red Flower-Like Plants
After entering the cave system you will probably see some outcroppings, quartz, and maybe even a creature's egg. However, you want to keep your eyes peeled for a red flower-like plant.
When they are open you will see a brownish-yellow substance inside and this is the Cave Sulfur. You can collect it without using any special tools, but it is best not to linger. However, if you come across a closed plant, then you need to approach it cautiously.
Beware of The Crashfish
The Crashfish are the creatures that live in the Cave Sulfur Plants, and they are aggressive but not nearly as dangerous as a Leviathan. If you enter a cave and it comes after you, then it will cause a chemical explosion that will damage you.
You know they are in the vicinity when you hear their tell-tale warning gurgle or loud screeches. The best way to collect the Cave Sulfur they are sitting on is to lure them out of their plant, then use the cave system to your advantage.
The Seaglide is great for this as it will allow you to quickly maneuver in and out of the twists and turns its home consists of. Once you have lost it, you can head back and collect your Cave Sulfur.
More daring adventurers don't mind taking some damage while collecting this resource and then they just use a Medical Kit to heal up afterward.