Inviting: Sinkholes like the stunning one in Oman are formed when groundwater travels through easily-dissolved rock
Visitors are regularly spotted at the Oman site taking a dip in the vibrant emerald-tinted waters of its 20m-deep pool.
Sinkholes are found worldwide, and can form suddenly or over time, ranging in size from just one metre wide to 600m in size.
The ground beneath the holes is normally made of easily-dissolved rocks such as limestone, carbonates and salt beds.
When groundwater flows through these rocks, it eats away at the rock, leaving behind subterranean holes and caverns.
When the roof of one of these caverns collapses, the land above it falls in too, leaving giant holes such as the one in Oman.