How do we know the Arctic is just ice?
The sea floor has been mapped using sonar from ships as shown here, and radar. But the fact was first discovered long before these technologies existed by sailors who got stuck in the ice. The entire ice sheet is floating and spinning at a relatively high rate of speed. When they got stuck, they could see, from the stars, that they were moving. I cannot find it now, but I remember reading on project gutenburg a terrifying historic first hand account of being stuck up there, and seeing that they were moving away from Europe and civilization and could not move their ship.