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Giant Seaweed can only be obtained in the Underwater section of Fossil Island. You may either grow it in the two Seaweed patches available, or kill Lobstrosities to try and get it as a drop.
As a result, completion of Bone Voyage is required for access to Fossil Island.
It’s also strongly recommended (although not required) to bring a Diving Apparatus and Fishbowl Helmet if you’ve completed the Pirate Pete section of Recipe for Disaster.
This is because your oxygen level will drain as you stay underwater. If you don’t refill it with pufferfish or bubbles (or just leave), you will eventually drown, considered an unsafe death.
But those two items completely bypass the oxygen mechanic.
Additionally, bringing Flippers (dropped by Mogres) will allow you to toggle Run and move faster around the area.
Start – Getting Underwater
In order to head underwater in Fossil Island, you’ll need to head north, past the Mushroom Meadow and near the Ammonite crabs.
There you’ll find a rowboat, which will take you to a small island (“Head out to the sea”).
On the small island there’s a bank chest, which is free and permanent.
Once you’re ready, click the boat again, and choose to go underwater.
Option 1 – Farming Giant Seaweed
Requiring 23 Farming, this is by far the easiest and quickest way to obtain Giant Seaweed in large quantities.
And this works as any other farming patch.
You’ll find a Tool Leprechaun and a farmer between the patches.
The seeds you need here are Seaweed Spores.
You might get them by trading with other players, pickpocketing master farmers, or purchasing them from mermaids (not recommended).
Additionally, they spawn in the Underwater floor randomly, in stacks of 1-3.
This is the lowest effort method of getting spores for Ironmen, so you might want to sit around as your patches grow.
Giant Seaweed takes 40 minutes to grow (4 cycles of 10 minutes) and yields at least 3 crops per seed.
It’s worth noting that Ultracompost increases yield and lowers the chance of disease, but Magic Secateurs have no effect.
The farmer will protect your patches for 200 numulite each.
It’s usually not recommended if you’re using Ultracompost, but it’s not too expensive if you’re in a hurry!
Once you’ve harvested your patches, simply plant another seed.
This would be a great time to train a bank skill since there’s a bank chest available just above, and Giant Seaweed doesn’t take long to grow.
Option 2 – Killing Lobstrosities
Lobstrosities are aggressive Level 68 enemies that attack only with Melee.
Among runes, herbs, and seeds, there’s a chance of 1/15 for them to drop 6 Giant Seaweed in noted form.
While strictly less efficient than Farming, Lobstrosities can be something to do while your patches grow, and get you a couple more seaweeds over time, given your combat level is high enough.
These enemies are found in the south-east corner of the Underwater section.
It’s worth noting that you cannot wear a Fishbowl Helmet in this area; Nevertheless, there are plenty of air bubbles to replenish your oxygen around Lobstrosities.
And since you’re underwater, you can only attack Lobstrosities with:
- Powered Staves (Tridents, Sanguinesti Staff, etc)
- Magic Secateurs
- Brine Sabre (only special attack available in this area)
The area is pretty narrow, so you can easily stand inside an air bubble and AFK for a while as they aggro, if your stats allow.
Once you’ve had enough, either teleport out or walk back to the surface. And that’s it!
Whenever you’re ready to tackle those Crafting levels, take off your shoes, grab that swimming gear, and get diving.