Can anyone guess what this small hole filled with water, in the center of a fort built in the 1600s on an island just off the coast of Colombia was used for?

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Answer:

Small narrow tunnels shot outward and connected to the sea. As boats approached they could tell which direction the boats were coming from by observing the water’s movement in the hole.

The local guides called it a sefon, and we made a joke it was a sea-phone, because it was a communication device for the sea. 

Pretty neat engineering design.

There were some good guesses in the replies.

Wowwww
This is actually innovative and smart 

They’d probably use it to know when enemies or travellers were coming by sea back then