Thunderground
Thunderground is a 1983 Atari 2600 cartridge from Sega, programmed by Jeff Lorenz. You drive a digging vehicle called a core capsule through the earth, tunnelling towards six enemy bases while two tanks hunt you through the passages you create. It borrows the tunnelling idea from Sega’s 1981 arcade game Borderline and adds a hidden bonus item to find in every round, which turns a simple shoot-out into a small treasure hunt. You can play it here in your browser.