It did not become a popular tourist destination until the early 1970s when small chartered passenger boats became more frequent; five were available in 1973 at a price of £3 per person. By 1990, the level of demand had grown to the extent that the Office of Public Works began to organise ten boats departing from four individual harbours. Each year at least four boat licences are granted to tour operators who run trips to Skellig Michael during the summer season (May to October, inclusive), weather permitting. Even when conditions on the mainland are calm, the sea around the island can be turbulent. The area is a landing point for sea swells travelling in from distant depressions in the Atlantic. The island is lashed by water from all sides, with wave crests breaking at up to 10 metres over the pier and 45 metres along the lighthouse. When such large wave troughs recede, they can often expose ragged seaweed stumps, sponges and anemones on the rock faces, further hampering approach and landing. A typical visit lasts about six hours. The Office of Public Works emphasises that the journey presents safety challenges. For safety reasons, mainly because the steps are steep, rocky, old and unprotected, climbs are not permitted during wet or windy weather. Dive sites immediately around the rock, mostly around Blue Cove, are permitted in summer.