The first images of the submerged caldera of the Tongan volcano that exploded in January have been released.

A robotically piloted boat has returned from its initial survey of the now underwater Hunga-Tonga Hunga-Ha’apai (HTHH) caldera – the ‘opening’ of the volcano – which erupted in January causing the deaths of four people. The eruption spewed ash ‘halfway to space’ in one of the most destructive geophysical events of modern times.

Previous research found the volcano surprisingly intact even though much of the former sea mount had disappeared below the surface of the Pacific Ocean following the January eruption.

The USV Maxlimer is a remote-controlled research vessel that is crisscrossing the sea mount in a bid to develop images of what is left intact below the ocean. Initial reports show that volcanic activity is ongoing.

The vessel is surveying the volcano as part of the second phase of the Tonga Eruption Seabed Mapping Project (TESMaP), led by New Zealand’s National Institute of Water and Atmospheric Research (Niwa) and funded by the Nippon Foundation of Japan. It is piloted from a control room 10,000 miles away in Essex, UK.

Ashley Skett, director of operations at Sea-Kit, the company that designed the USV Maxlimer, told the BBC: “The boat has been designed from the ground up to be remotely controlled and remotely operated. So every switch, every function on the boat, every light, we can control from here.”

USV Maxlimer is due to return to mapping duties shortly to complete the images that will help scientists better understand what happened in the run up to the eruption and whether there is likely to be more volcanic activity going forwards.