Simplify the following text for a Grade 4 native English-speaking person.
Original: The tunnel , which was about 1.2 m high by 0.9 m wide , extended more than 1000 m beyond the Military Demarcation Line ( MDL ) into South Korea .
Simplified: The tunnel , which was about 1.2 m high by 0.9 m wide , extended more than 1000 m beyond the MDL into South Korea .