Probably by now everyone has read of this discovery, which may date to around 2900 before present. From the BBC:
The stone slab, named the "Cascajal block", was first uncovered by road builders digging up an ancient mound at Cascajal, outside San Lorenzo, in the late 1990s.
It weighs about 12kg (26lbs) and measures 36cm (14in) in length, 21cm (8in) in width and 13cm (5in) in thickness. Its text consists of 62 signs, some of which are repeated up to four times.
The team says the text "conforms to all expectations of writing" because of its distinct elements, patterns of sequencing, and consistent reading order.