The climate tech startup Running Tide has a unique approach to carbon removal that involves wood waste, seaweed, and the deep ocean.

On a chilly day in the middle of May, a boat sailed from a port on the west coast of Iceland towing unusual cargo: a raft filled with millions of tiny wooden buoys, each no larger than a golf ball. Nearly 200 miles off the coast, the crew started to drop the buoys into the water.

The expedition was a pilot test for Running Tide, a climate tech startup that plans to later use similar floating “carbon buoys” to grow seaweed in the open ocean—and then sink both the buoys and the seaweed to the bottom of the ocean to store the carbon that they contain. The test of the buoys in May, like others that followed in June and July, was designed to “get us to the starting line of actually being able to run a negative emissions supply chain,” says Running Tide CEO Marty Odlin, meaning that it will sell carbon removal services to customers that want to offset their own emissions.