A rotary dial telephone set and clock are seen in the NATO tunnels dating back to the Cold War in the War Headquarters tunnels beneath Valletta, Malta, March 28, 2017. A bathroom sink is seen in the War Headquarters tunnels, parts of which date back to World War Two, beneath Valletta, Malta, January 26, 2009.REUTERS/Darrin Zammit Lupi SEARCH "LUPI TUNNELS" FOR THIS STORY. REUTERS/Darrin Zammit Lupi SEARCH "LUPI TUNNELS" FOR THIS STORY.
They originate from processes deep within the Earth—typically at depths of about 50 to 200 kilometres (30 to 120 miles)—in the mid- to lower-crust or in the upper mantle.
Sedimentary rocks are generally stratified—Metamorphic rocks are those formed by changes in preexisting rocks under the influence of high temperature, pressure, and chemically active solutions.
The changes can be chemical (compositional) and physical (textural) in character.
Malta is now restoring the 28-km (17-mile) of tunnels, planning to open a huge section to the public.
The compound, hidden under the picturesque port city perched on cliffs above the sea, was built by the British and served as the staging ground for major naval operations.