A US teenage girl sailing solo around the world is thought to be in distress in a remote area of the Indian Ocean.
Californian sailor Abby Sunderland, 16, was last heard from at about 1300 GMT on Thursday before emergency beacons were activated, her brother said.
She is believed to be more than 3,220km (2,000 miles) from the coasts of both Africa and Australia.
Search vessels are now heading towards her, but officials say they will not arrive for another two days.
An aircraft was scheduled to depart from Perth, on Australia's western coast, on a four-hour flight to Ms Sunderland's location, Australian Maritime Safety Authority spokeswoman Carly Lusk said.
Ms Sunderland's brother - who also circumnavigated the globe solo, aged 17 - paid tribute to her skills but voiced concern at her lack of radio contact.
"The distress beacons mean she's in some kind of trouble. She's a very accomplished sailor but she's in the Indian Ocean right now and it's a really dangerous place," Zac Sunderland said.
But he added: "She's got all the skills she needs to take care of what she has to take care of, she has all the equipment as well."
In a post on the teenager's blog, her family wrote that she had battled 60-knot winds and 20-to-25ft (6-7.6m) seas before going missing.
Zac Sunderland completed his own solo voyage around the globe in 2009 at the age of 17.
His sister set sail from California in January, attempting to beat her brother's record and become the youngest person to complete the journey.
She had to give up the goal of setting the record after stopping in April at Cape Town, South Africa, for repairs to her boat, named Wild Eyes.
But she later decided to continue the voyage.