The Romanian town of Costesti is home to one of the world's most amazing natural phenomena - rocks that grow. Named Trovants or 'cemented sand' the spherical shaped stones are believed to have first appeared on earth about six million years ago, following a powerful earthquake.

At first sight, Trovants that comprise of a hard core and sandy shell, appear to be like any other rocks. The magic begins when it rains. As soon as the last drop dries out, small formations start bulging from the sides - almost as if the stones have come alive.

And the growth is not minuscule either. Researchers claim that stones that were once a mere six to eight millimeters often end up measuring six to ten meters wide. As they get larger, Trovants tend to split from their 'birth mother' and continue the magical growth process, independently. What's even more amazing is that when cut horizontally, Trovants reveal growth rings that are similar to ones found in the cross section of tree trunks.

The strangest part is that despite having known about them for many years, scientists have no idea how Trovants 'grow'. One of the most plausible theories is that the growth is caused by the high mineral content present in the rocks. Researchers speculate that when exposed to moisture, the minerals expand and in turn put pressure on the sandy shell, forcing it to jut outward and making the stones 'grow'. The other mystery of course is why these rocks are found only in one area of the world.

Not surprisingly, the unusual rocks have become superstars and the Muzeul Trovantilor museum dedicated to them is visited by millions of tourists every year. In order to protect them for future generations, the area was declared a national preserve in 2004 and is also one of the 193 natural sites protected by UNESCO.,,