Complex sentence: Only a small percentage of the German fortifications are open to the public due to the tragic deaths of souvenir hunters through lack of oxygen, many such as the large tunnels complexes (similar to Ho8) are now sealed off as a result.
Split into simpler sentences: Only a small percentage of the German fortifications are open to the public, due to the tragic deaths of souvenir hunters through lack of oxygen. Many of the large tunnels complexes (similar to Ho8) are now sealed off as a result.