Just days after the sighting of around 2,000 dolphins off the west Wales coast, a school of giant fin whales has been spotted fishing in the Irish sea.
Scientists said the biggest of the whales was 40ft
The sighting by an Oxford University team was described as "unique" as they are normally on their own or in pairs.
Zoologist Dr Peter Evans said the sea "teeming with food" has put west Wales on the whale watching map.
"It was an experience of a lifetime. I see whales all around the world but this was really spectacular."
Steve Lewis whose safari company ran the trip, added: "These huge animals are normally seen singularly or in pairs.
"This is the biggest sighting of fin whales ever spotted in UK waters.
"The boat we were in was 35 feet long, and the biggest of the whales was bigger than that. It must have been 40ft plus.
"For the UK this a unique experience. There's no record of them being seen in these numbers before."
The fin whale is the second largest animal on the planet after the blue whale.
They are born at 21ft (6m) and can grow to be 85ft (26m) in the Antarctic. They weigh between 30-80 tonnes and at this time of year consume up to 35 grams of food for every kilogramme of body weight - every day.
Experts say it is that which holds the key to their arrival off the coast of west Wales.
Dr Evans, from the zoology department of Oxford University, leads the Sea Watch Foundation expeditions to Pembrokeshire.
Rich stocks of mackerel are attracting whales and dolphins
Describing the Irish Sea as "teeming with food" this summer, he explained that it was large schools of mackerel and herring which are attracting the unusual numbers of larger visitors.
"Everywhere you look there are fish," he said.
"When we were out we were surrounded by thousands of sea birds, gannet and Manx shearwaters, all feeding in the same area."
The fin whales have been the third unusual marine sighting reported in West Wales in two weeks.
At the weekend a group of up to 2,000 common dolphins was spotted, which marine experts described as "massively unusual."
And last week two humpback whales were seen, 100 metres off the beach at Llangranog.
Steve Lewis said the sighting was "a unique experience"
"We have seen unusual numbers of minke whale too," he said. "We often get one or two, but this week we've have seen up to 10.
"The increased wildlife may be because of changes in the currents off our coast," he added. "The reverse change is taking place in Scotland where the spawning grounds for sand eels and sprats are failing."
It is the sand eels that attract the mackerel and herring and the mackerel, herring and plankton that form the diet of the fin whales.