The mines are dropped in the sea around Macross Island. (Robotech/Voltron: 4)
Gravity Mines were advanced gravitational devices that were used by the Zentraedi in their military campaigns. They were typically deployed underground and were able to reach as far as three kilometers beneath the surface. Once placed, they accumulated energy and worked by counteracting the effects of anti-gravity drive units preventing them from functioning.
During the First Robotech War, the 7th Mechanized Space Division of the Botoru Fleet under Captain Khyron's command were ordered by Commander Breetai to engage the SDF-1 on Mars. Their mission was specifically to capture the battle fortress through the use of gravity mines. Thus, Geraro was deployed in a Battlepod to place the mines beneath the site of the SDF-1's projected landing zone at Mars Base Sara.
Ultimately, Captain Henry Gloval fell for the trap and discovered that the damage to his ships engines prevented the SDF-1 from retreating into orbit due to the gravity mines working against the anti-gravity units. However, he had Lisa Hayes overload the underground Reflex Furnace of Sara Base which exploded with such force that it knocked out the Zentraedi mines whilst also devastating Khyron's assault force. (Macross Saga: Bye-Bye Mars)