Coolant pours out of bottom of engine - 2000 Nissan Sentra

Are temperatures below freezing where you live? Is the vehicle kept outside? Cracked engine block, or if you are lucky a freeze plug has pushed out. Or a split hose.

https://www.google.com/search?q=2007+Nissan+Skyline+coolant+is+pouring+out+of+back+of+engine

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The cooling system?
Loosen the bottom rubber radiator hose and let the coolant run out.

Buy a can of coolant system flush. Reconnect the bottom radiator hose, pour the can of coolant flush in and top up with plain water. Drive the car for about 10 minutes until the engine is warm.

Then remove the bottom radiator hose and let the water run out.
Reconnect the bottom hose and fill the cooling system with coolant/water mix. Run the engine for 5 minutes and then check the coolant level

Do you have a 4 or 6 cylinder engine?

The water pump is enclosed on the front of the engine. The bottom radiator hose connects to the water pump inlet.
You do not add coolant to the water pump. You add a 50-50 mix of antifreeze and water into the radiator cap opening. Or you can buy the coolant already mixed with water and just pour it into the radiator.

The coolant system uses the water pump to circulate the coolant through the engine and then into the radiator via the upper radiator hose for cooling it off, and it then goes back into the engine through the bottom radiator hose. When the engine is warmed up, the coolant constantly circulates through the engine to radiator and back into the engine.

see if its coming out from the bottom of the engine where the water pump would leak

Sure could be. Or a hose or a gasket somewhere.
You'll need to fill up the radiator and run the engine to look for the leak.
It may have to get up to operating temp before you see anything.

you need to replace thermostat and radiator cap.look under hood around engine compartment.look for a radiator cap near radiator,add 50/50 antifreeze and water to coolant system to bring up boiling point.pure water will cause engine run hotter because water evaporate away,remove radiator pour coolant in the radiator until coolant stop dropping in the radiator now some vechicle pour coolant in the coolant overflow jug,pour coolant until coolant level stop dropping and the coolant level stay at cold full mark.start engie let idle few minutes, watch coolant level in radiator if coolant start dropping add more coolant, when coolant level stop dropping put radiator cap back on let engine idle until both top radaitor hose and bottom coolant hose get warm turn off engine wait until it get cool then take a large rag slightly open radiator just a litle at a time to keep from getting scaled.once radiator remove add more coolant if low,put radiator cap back on start the engine let it idle watch the temp gauge if temperature gauge going hot zone stop engine wait until cool then add more coolant,keep doing this until engine stop overheating going into hot zone and coolant level stop dropping,make the coolant overflow jug coolant level stay at full cold mark, dont let jug run empty if so air will get in the coolant system causing engine to overheat.

Well, i would have to say all of the above is a direct result of the water pouring out of the bottom. your coolant system must be full to function correctly, by the cylinders heating the coolant to keep them cool, and then the coolant is cooled back down in the radiator. you can damage the cylinders and more if you run it like that. look for exactly where the water comes out when you pour it in (try to contain any coolant. water alone would be fine to let run on the ground) my hunch is that you probably had a freeze plug come loose from the block, or blew a heater or radiator line. a freeze plug is the little 2" metal circle plate (or even block heater) pressed into the side of the engine under the exhaust on both sides. another possibility could be the water pump. this would be coming from the front side of the engine. there are also many other leak sources, but these are the most common. check the oil. if it is creamy, then there is coolant going into the engine. a freeze plug is an easy fix, as they sell rubber expansion plugs to plug the hole. you sinply stick it in and tighten the nut/screw as per directions and done. a blown hose is relatively simple to fix, but can also be complicated, depending where. most of the heater connections have o-ring and plastic clips that tend to be difficult at times to dis-assemble. a water pump would be expected to fail anywhere between 100k-150k miles typically, and is a cheap repair (i would expect 2-300 in my area)

WAIT UNTIL YOUR ENGINE IS COLD,YOUR RADIATOR PRESSURE CAP IS LOCATED ON THE COOLANT OVER FLOW RESERVOIR, THATS WHERE YOU ADD COOLANT WHEN COOLANT LOW,YOU POUR COOLANT IN THE OVER FLOW RESERVOIR UNTIL COOLANT LEVEL REMAIN AT FULL COLD MARK DONT OVER FILL OVER FLOW RESERVOIR, WHEN COOLANT LEVEL STOP DROPPING WHEN ADDING COOLANT AND COOLANT LEVEL STAY AT COLD MARK ON THE RESERVOIR YOU RADIATOR HAS THE CORRECT COOLANT LEVEL IF COOLANT LEVEL KEEP DROPPING CHECK FOR COOLANT LEAKS AROUND RADIATOR AND THE TOP AND BOTTOM RADIATOR HOSES, LOOK FOR COOLANT LEAK AT WATER PUMP GASKET OR SEAL OR LEAK AT WATER PUMP WEEP HOLE IF YES REPLACE WATER PUMP.

This is how I like to do it. Make sure the car is parked with the front end at an upward angle. Drain the coolant from the petcock at the bottom of the radiator. Remove the upper and lower hoses at the radiator. Remove the thermostat housing, and the heater core hoses at the fire wall. Flush out the radiator by pouring fresh water into the overflow, and the hole where the thermostat was. Flush the heater core as best you can by pouring fresh water into the heater core hose connections.
Let all the water drain out, then button everything back up and replace the thermostat with a new one. Refill the rad and engine by filling the coolant reservoir with a mix of 50% water and 50% antifreeze, or buy the premixed stuff from the auto parts store. (Buy 4 jugs of premix or 2 of straight antifreeze)
Run the engine until it reaches operating temperature and the upper rad hose gets hot. Rev the engine a little to help get the air out of the cooling system. When the thermostat opens, the coolant level will drop in the reservoir, so top it up and replace the cap.
Check for leaks, and check the coolant level for the next few days, and check for leaks.
Dispose of the old coolant responsibly