In the relatively flat Harman Valley, located between Wallacedale and Byaduk, south of Mount Napier in Victoria, Australia, are peculiar rocky mounds, like blisters on land. Some of them are up to 10 meters high and 20 meters in diameter. These mounds are known as tumulus or lava blisters.

Tumulus are formed in slow-moving lava fields. When lava flows, the surface often cools to form a thin crust, but underneath the lava is still viscous and molten. If the advancing lava underneath becomes restricted it may push up on the hardened crust, causing soft spots in the crust to rise up like a bubble. Generally, these structures grade into elongate forms called pressure ridges, but occasionally, they creates smaller, steep-sided domes called a tumuli. Usually, the dome is completely solid, but occasionally, part of the liquid core drains out and the top of the dome subsides to leave a central hollow or doughnut-shaped mound.

A tumuli near Byaduk, in Victoria, Australia. Photo credit: Lesley A Butler/Flickr