Researchers have been studying the Amazon rainforest for more than three centuries, and there’s still so much they don’t know. A new study estimates that there are nearly 12,000 species, with thousands more yet to be discovered and documented.

The study published in the journal Scientific Reports is the first attempt to focus on tree diversity, say the researchers. There have been regular attempts to maintain collection records of birds and mammals, but not of plants.

“Before this paper we didn’t have a list of Amazonian trees,” said Nigel Pitman, a tropical forest ecologist at the Field Museum in Chicago, and an author of the study, according to the New York Times. “With this list we are answering ‘How many species have been found?’ and ‘What are they?’”

In 2013, the study’s lead researcher Hans ter Steege and his colleagues estimated that there were approximately 16,000 tree species living in the Amazon. “After a while, I was just curious how many species of tree had actually been collected in the area,” he told Live Science.

After analyzing more than 500,000 specimens dating back to 1707, Pitman and his team released a checklist containing 11,187 valid species names. In addition, 489 valid species names known to occur in Amazonia, but whose data could not be found in the records, took the tally to 11,676.

“We interpret this to mean that our 2013 estimate of 16,000 species is good, and that about 4,000 of the rarest Amazonian trees remain to be discovered and described,” Pitman said in a statement. These rare species may take the botanists centuries to track down, Steege said.

“Since 1900, between 50 and 200 new trees have been discovered in the Amazon every year,” Pitman said. “Our analysis suggests that we won’t be done discovering new tree species there for three more centuries.”