Lerwick is a name with roots in Old Norse and its local descendant, Norn that was spoken in Shetland till the mid-19th century. The words Ler Wich mean muddy bay. The corresponding Norwegian name is Leirvik. Leir means clay and vik means bay. All year, its sheltered harbour at the heart of the town is busy with ferries, fishing boats, oil-rig supply vessels and a variety of more specialized craft including seismic survey and naval vessels from all round the North Sea. In summer, the quaysides come alive with local pleasure craft, visiting yachts, cruise liners, historic vessels such as the restored Swan , and the occasional tall sailing ship. Behind the old harbour is the compact town centre, made up of one long main street, Commercial Street; from here, narrow lanes, known as " closses ", rise westwards to the late-Victorian new town. Lerwick's attractive, flagstone-clad Commercial Street is still very much the core of the town.