I had Nagamo soba at the Naoetsu Terminal Restaurant located in the Sado Kisen Naoetsu Terminal.

Nagamo is a specialty of Sado Island, known for its strong stickiness, which pairs perfectly with soba noodles, making them slide down easily. I ate at an outdoor table, and the clear blue sky and pleasant sea breeze enhanced the atmosphere.

The following is quoted from the Niigata Prefecture website (https://www.pref.niigata.lg.jp/):
Nagamo, whose standard Japanese name is Akamoku, is a large seaweed that can reach a length of 3 to 10 meters. It has air bladders in its thallus, and uses this buoyancy to grow upright. It is a seaweed that grows in the winter when the water temperature is low, and in Sado, it is harvested from the end of December to around March.

Currently, Niigata Prefecture is conducting Nagamo cultivation trials on Sado Island to ensure a stable supply of Nagamo. Nagamo is a seaweed characterized by its unique stickiness. It is mainly eaten in miso soup and vinegared dishes.

To prepare Nagamo, scrape the leaves from the stems, place them in a colander, and blanch them in boiling water. The seaweed will turn a vibrant green when cooked. After cooking, cool it with cold water. Nagamo can be stored for a long time by freezing it after cooking. If you boil a large amount, you can portion it out and freeze it for later use.

Sado Kisen Naoetsu Terminal Restaurant
1-9-1 Minatomachi, Joetsu City, Niigata Prefecture
Google map: https://goo.gl/maps/AeDHGfUxg3qBwu8PA