You'll need to find Monster Hunter Rise King Rhino, Rock Roses, and Bismuth Prisms to complete the Economic Stimulation request and unlock the third submarine at the Argosy merchant in the village. This quest is unlocked later in the village campaign, but it's every bit as vital as the Monster Hunter Rise Wisplanterns and Boatshells request that adds a second submarine. It's also just as difficult to complete because the items it requires are hidden around several maps, but once you know where to look, it only takes a few minutes of gathering.
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Where to find Monster Hunter Rise King Rhino, Rock Roses, and Bismuth Prisms
King Rhino, Rock Roses, and Bismuth Prisms are all rare materials found at specific gathering spots. In other words, you won't always get these items when you gather at these spots, but it doesn't take too many tries to get them. You need one King Rhino, three Rock Roses, and three Bismuth Prisms to complete Economic Stimulation. All of these items can be gathered on low rank expedition tours in their respective maps, which we'll go over one-by-one.
Monster Hunter Rise King Rhino
We'll start with the King Rhino, which is found in the Flooded Forest. When you load in, head southeast to the huge temple at the bottom of the map. You can use normal wirebugs or the giant wirebug anchor on the stairs to scale the temple. Near the top – but not at the absolute peak – you'll find a door which can be blown open using a large barrel bomb. Place a large bomb to break the door and a small bomb to detonate the large one.
Once the door is open, look for a gold beetle on the right wall. This gathering point has a chance to drop a King Rhino, but it's not guaranteed. If you don't get a King Rhino, reload your expedition tour and come back to this spot. Just note that you will need to blow the door open again, so bring extra bombs. You can buy bombs from the merchant if you need more.
Monster Hunter Rise Rock Roses
Next up, Rock Roses. These can be gathered in the Sandy Plains. If you have sub-camp two unlocked, fast travel to it when you load in. (If you don't have the camp, you'll discover it as we go!) Head to the hideaway south of area nine and east of area eight, pictured on the map above, then look for a small hole on the northeast wall. Drop down this hole, turn around to face the main cave area, and look to your left.
You'll see a pile of round stones on a rock below. This is the gathering point which gives Rock Roses. If you don't get enough Rock Roses from one pile, you may need to wait for it to respawn (just a few minutes – plenty of time to gather the bones and ore around) or load another expedition tour to finish off the Rock Roses.
Monster Hunter Rise Bismuth Prisms
Finally, let's go over where to find Bismuth Prisms. These are located in the Lava Caverns, and they're the easiest resource to get for this request. You'll save some time if you have sub-camp one or even two for the Lava Caverns unlocked, but it's easy to reach the Bismuth Prisms regardless. Just head to area 10, which is an elevated magma plateau off to the right.
Once you're up top, look for some viridescent purple rocks on the north side. Mine these to collect Bismuth Prisms. Same deal with this gathering point: if you don't get enough in one go, you can wait for the rocks to respawn or just reload an expedition tour. If you don't have it, be sure to stop by sub-camp one north of the rocks, too.
Tada! Once you've got the resources you need, the side quest Economic Stimulation will automatically be completed, so you'll just need to revisit the Argosy merchant to start using your third submarine. Enjoy those extra materials!