Roopkund, a high-altitude glacial lake in Uttarakhand, is popularly known as the:

Roopkund is a small glacial lake in the Chamoli district of Uttarakhand, at a height of about 5,020 metres, near the base of the Trishul massif. It is called the "Mystery Lake" (or "Skeleton Lake") because hundreds of human skeletons have been found in and around it.

Why it is called the Mystery Lake:

• In 1942, a forest ranger reported finding a large number of human remains around the lake.
• The remains become visible when the snow melts in summer.

For many years, their origin was unclear. Popular explanations included a group of pilgrims, soldiers or travellers caught in a disaster.

One widely reported theory suggested that many of the people were killed by a severe hailstorm, based on head injuries found on some skulls. This is linked to a local folk song about a goddess sending hailstones on a group that offended her.

Later DNA and radiocarbon studies (published in 2019) found that the skeletons belonged to different groups of people who died at different times, some as early as about the 7th to 10th centuries CE and others roughly 1,000 years later. One group appeared to have eastern Mediterranean ancestry. The exact cause of their deaths is still debated, which adds to the "mystery".

About the lake and trek:

• The lake stays frozen for much of the year.
• It lies on the route of the Nanda Devi Raj Jat Yatra, a pilgrimage held about once every 12 years.
• The Roopkund trek passes through Lohajung, Wan, Ali and Bedni Bugyal (high meadows).
• Because of concern for the remains and the fragile environment, trekking right up to the lake has at times been restricted.

Why not the other options?

• ✗"Lake of No Return" is the popular name of the Nawng Yang Lake near the India-Myanmar border, associated with stories of aircraft lost during the Second World War.
• ✗"Rainbow Lake" and "Twin Lake" are not standard names for Roopkund.