Scientists landed on the three-year-old land mass to find unusual sticky mud and plenty of vegetation growing on it.
NASA scientists are baffled at the unusual sticky substance covering the world's newest island after making their first visit.
The volcanic island was formed after an underwater eruption in early 2015 near the South Pacific island of Tonga.
It has been studied and mapped for years using satellite technology, but no one from NASA had stepped foot on it before.
The South Pacific island is one of three new islands to form in the last 150 years and survive the rigours of the ocean for more than a few months.
Scientists have kept a close eye on the island, which could provide an insight into how new islands form and evolve on earth and how volcanic landscapes interact with water on Mars.
But after Dan Slayback of NASA's Goddard Space Flight Center and a team approached the island - which posed unexpected steep beaches and rough waves - they noticed the landscape was different to what satellite research showed.
“We were all like giddy school children,” Mr Slayback told a NASA blog.
“Most of it is this black gravel, I won’t call it sand – pea-sized gravel – and we’re mostly wearing sandals so it’s pretty painful because it gets under your foot.
"Immediately I kind of noticed it wasn’t quite as flat as it seems from satellite. It’s pretty flat, but there’s still some gradients and the gravels have formed some cool patterns from the wave action."
But it was the mysterious mud that baffled the team.
"And then there’s clay washing out of the cone. In the satellite images, you see this light-colored material," he added.
"It’s very sticky. So even though we’d seen it we didn’t really know what it was, and I’m still a little baffled of where it’s coming from. Because it’s not ash."
Another surprise was the vegetation scattered across the island, most likely a result of bird droppings on the volcanic cone's base.
The goal of the expedition was to collect rock samples and to record the elevation of the island.
The data might help determine the island's volume and how much ash and volcanic material erupted from the vent along the rim of the submarine caldera below.