Quatermass and the Pit (1958)

When a skull is found during building works at Knightsbridge, London, the work is halted so a full archaeological dig can proceed. The diggers delve deeper, finding more skulls, but also finding some form of tube-like shell made of a ceramic like material. The Ministry of Defence believe that it's an unexploded bomb, but when they manage to dig inside the shell, they find dead insect-like creatures. The MOD continue with their story, but Professor Quatermass' theory that the insects are Martians who visited Earth over five million years ago is proven correct with drastic consequences.