Since the Martians in this scenario are such good tunnelers, when Mars actually lost its surface water or atmosphere isn't all that important.
Perhaps they have been living a subterranean existance for centuries, even eons. They might live just below the surface, sort of like Hobbits, scanning the heavens with their surface telescopes while major cities thrived just meters below, powered by energy tapped from the Martian core. All the water they needed has been derived by melting the polar ice caps from below.
Then something went terribly wrong. For 1000s of years, the Martians have been carefully monitoring the size of their ice cap. It would go through periods of growth and shrinkage, in a series of delicate cycles. But, after the most recent cycle of shrinkage, instead of recovering the cap continued to shrink, and in the year [1906], Martian scientists calculated that their polar cap, the life blood of Martian civilization, had passed a tipping point. It would not recover, and it would continue to shrink.
As the cap got smaller, there was less ice surface area to reflect the sun, and so the sun's rays were absorbed into the planet, making it warmer, causing the cap's shrinkage to accelerate in a now irreversable death spiral. Martian civilization was doomed. Why this had happened was never fully understood. Perhaps changes in the amount of energy from the sun, or perhaps the effects of Martian industry had a small but meaningful impact on the planet's temperature. It no longer mannered. What did matter was that Mars was dying, and so would the Martians if they could not find a solution.
Still, there was time — perhaps maybe 150 years or more before the cap would disappear altogether. So it was, that in [1929] their great and inspiring leader Ydennek rallied the Martians to the cause. "It is our goal that by the year [2006], we will put Martians on the [Earth]." Martian scientists and industry rose to the challenge, and made much progress.
But five years later, they made a disturbing discovery. The Martians always knew there was water and plant life on Earth. They could see that much with their powerful telescopes, and without hindrance of a thick atmosphere. However, as they pushed above the surface and begain scanning their blue neighbor with more sophisticated equipment, they detected signals and signs that the inhabitants of earth were not only intelligent, but also, though they could not be certain, possibly belligerant.
Still, the Martians had no choice. They had to proceed. So they devised a plan. They would send ahead a small group of Martians to scout out their new home, and to pave the way for the waves of Martian refugees. The Earth's south pole (Antarctica) was the perfect location for such an outpost. The climate was just like home, even warm by Martian standards, and there was a huge ice sheet through which they could tunnel, which was situated over a solid landmass for stability, and down through which they could use their laser borers to tap into the Earth's core for all the energy they would need.
And so, in [1951] three dozen Martian scout/colony ships set out. The journey took over a year, and the Martians had little experience in interplanetary travel. As a consequence, few ships actually made it to their destination. Many were lost soon after launch and many failed during the transit. When the fleet reached earth, there were only seven ships left. Two of these burned up in the Earth's thick atmosphere. Two more suffered mechanical failure and crashed, one in the largest body of water on the planet, and the other some 2000 miles to the east, in a mountainous desert area. Niether could be recovered.
The last three ships were successful. They landed near each other in the planned location. Within minutes, the intense heat of their outer shells melted a new home for them through a hundred feet of ice.
The crews begain tunneling, and soon a small, subterrainean, interlocking complex was built. The Martian geologist started the laser core driller. The Martian geneticist placed the embyos they had carried for millions of miles into the incubators. The mission commander transmitted the first of many short, encoded messages. He received no answer. He knew there would be none. The next friendly communication the scouts would receive, would be from the fleet, when it had arrived. It did not matter. He had his orders.
discovers a lost dinosaur.
