A vessel for sequestering carbon includes a cavity associated with a first opening and a second opening. The vessel is configured to perform a first operating mode to collect seaweed into the cavity and a second operating mode to discharge the seaweed collected in the cavity for sequestration. In the first mode of operation, the vessel is moved across the sea surface, such that water and floating seaweed pass into the cavity via the first opening. The second opening is provided with a filter, such that seawater is able to exit the cavity via the filter, but the seaweed is retained by the filter in the cavity. In the second mode of operation, the vessel is submerged to a transition depth at which the collected seaweed is compressed by water pressure to have a density greater than the sea density. The collected seaweed is then discharged from the cavity to sink to the seabed for sequestration.