Marine Research Lab

The Marine Research Laboratory was a scientific facility located near the coastline in the Great Ocean in Termina, as well as on the shores of Lake Hylia in Hyrule. It was run by a similar looking old man.

Termina
During The Legend of Zelda: Majora's Mask, the laboratory was used as a base by Link while he recovered Lulu's eggs from the depths of the Great Ocean as Mikau.

Hyrule
In The Legend of Zelda: Ocarina of Time, the lab plays a much smaller role than it did in Majora's Mask, except for holding two Pieces of Heart. For one Link had to climb the lab, the other he had to dive to the bottom of the tank inside. For this, he required the Golden Scale.