They claim holes left in the sea bed from ships extracting gravel and sand are being refilled with sand from the shoreline of their local beaches.

BBC: Dredging 'causing erosion' claim

Opportunity has been trundling around Mars ever since, though it was designed for only a ninety-day mission. (Spirit got stuck in a sand trap in 2009 and quit transmitting nearly a year later.) It never found the lake bed, or organic material of any kind.

NEWYORKER: The Martian Chroniclers