Filming the The Secret World of Bats took Merlin to 5 continents and 9 countries in one year. That year was 1989, when he returned to Thailand for the second time. With a film crew headed by Dieter Plage, they told the story of guano mining at Khao Chong Pran and Rakang Caves. The rigorous travel schedule and time zone changes nearly killed Merlin.
This is what Wikipedia says about Dieter Plage who accompanied Merlin on the project and died shortly thereafter:
“Dieter was lead cameraman on a major Survival series shot in the Galapagos Islands in the 1980s and then tackled probably his most challenging assignment to shoot The Secret World of Bats. Filmed on five continents, the programme was the supreme test of Plage’s considerable camera skills. Working with one of the world’s leading bat experts, Merlin Tuttle, he explored in detail the amazing behaviour of bats and their vital role in the planet’s ecology. He had just returned to Sumatra to update the story of the orangutans’ battle for survival when he met his death while filming above the forest canopy.”