Lavernock (Welsh: Larnog) is a hamlet in the Vale of Glamorgan in Wales, lying on the coast 7 miles (11 km) south of Cardiff between Penarth and Sully, and overlooking the Bristol Channel.

Following overland tests at Salisbury Plain during March 1897, on 13 May 1897, the Italian born and recently British based inventor, best known for his development of a radiotelegraph system, Guglielmo Marconi, assisted by George Kemp (who was a Cardiff based Post Office engineer) transmitted and received the first wireless signals over open sea between Lavernock Point and Flat Holm island.

The very first message transmitted in morse code was "ARE YOU READY". This was immediately followed by "CAN YOU HEAR ME" to which the reply was "YES LOUD AND CLEAR". The morse recording slip for the first message is on display in the National Museum of Wales.

Following the initial opening exchange there followed detailed technical messages in both directions indicating each end's equipment settings and receiving sound levels. Marconi indicated that he was using a 20-inch (510 mm) spark on his equipment.

The successful test followed several days of trials and failure while adjustments were made to aerial length. Extensive trials were carried out over the remainder of the week in various weather conditions and with different settings on the equipment at each end. Marconi benefitted from the active encouragement of then Mr. William Preece (later Sir William Preece) who was Engineer-in-Chief of the Post Office and had himself transmitted radio telegraph morse signals across Coniston Water eight years earlier. Preece had been previously acted as a consultant to the Bristol Corporation's Electricity Department between 1883 and 1893. The Post Office engineers, including George Kemp who kept a detailed diary of these events, had been experimenting for some months at Lavernock Point. Kemp recorded the following in his diary of the experiments:

Marconi's new equipment was therefore used in conjunction with that already adopted by the Post Office. The initial tests were so successful over the three and a third miles (5.4 kilometres) stretch of water that it was quickly decided to relocate the telegraph equipment from Flat Holm to Brean Down Fort, near Weston Super Mare increasing the distance to nearly ten miles (16 kilometres) from the Lavernock Point transmitter.

Following these successful trials, Marconi subsequently vested his new patent rights in his 'Wireless Telegraph and Signal Company', which unfortunately prevented any further co-operation with the Post Office engineers. However, George Kemp immediately resigned from his Post Office position and joined Marconi's new company as head of engineering development.

In 1948, to mark the 50th anniversary of the experiments, a bronze plaque was unveiled by the Cardiff Rotary Club inside the courtyard of the recently closed church of St.Lawrence, Lavernock, commemorating the historic radio transmissions over nine miles (14 km) of open sea. The small stone hut that Marconi used to contain his experimental radio telegraph equipment still stands on the cliff edge at the end of the lane near Lower Cosmeston farmhouse.