The tunnel is actually designed to leak. As Eurotunnel explain on their website, 'seawater from the rocks above the tunnel drips through and is then pumped away.
The mountain material is solid and strong enough not to collapse the hole, or the hole is lined with reinforcement that transfers load from one side to the other. This is now iron/steel rings or reinforced concrete. Tunnel boring machines build the liners as they progress, and push forward using the last ring.
If you are travelling on LeShuttle, which uses the Channel Tunnel, you stay in your car during the 35 minute journey from Folkestone to Calais. Once your car is loaded onto the deck of its shuttle and your crossing starts, you can leave your car to stretch your legs or use the on board toilets.
You can pay a lot to just turn up without a booking. There are plenty of crossings (up to 4 per hour) and it runs all night, so it is never fully booked for days on end, though you might have fewer choices of departure time still available when you do book. Guess your arrival time as best you can.
Given that many tunnels are below ground, they have long been designed and built to be watertight and with features that provide flood defence. Anti flooding measures include drainage systems, pumps, waterproofing of walls and ceilings, and increasingly sophisticated flood warning systems.
Accidents in tunnels are more dangerous than in the open air. An explosion or fire may cause considerable damage and hazardous substances cannot be removed quickly. Smoke causes poor visibility, lack of oxygen rapidly occurs and there may be panic. It is difficult to escape from a tunnel.
Hazardous Articles include but are not limited to fireworks, firearms, explosives and weapons of any kind. 3.1 The Carriage of Hazardous Articles is subject to prohibitions and restrictions. 3.1. 2 Firearms: the Carriage of Firearms is RESTRICTED.