Pillow lava
Pillow lava is formed underwater, on the sea-bed. When the lava comes out and hits the sea water, the outside cools and freezes pretty much immediately, while the inside keeps on flowing. That means that it forms tube, which lengthens and widens until the pressure at the inlet end breaks open the tube and starts a new one. So you get the tubes interlocking. This is how La Palma grew…